how do you know that a neck is bent (up or down bow) or not?

Simply lay a ruler over the frets. You'll either see a small gap between neck and ruler, at about the 7th fret (which is what you want to see) or the ruler will rock slightly, meaning that you have the wrong kind of relief in the neck. The other thing that you may see is that there is no gap and no rocking motion, which would indicate a totally flat neck, which is also not desirable.
If you don't have a ruler handy, just fret the low E at the first and the 14th frets - the string will give you a straight line.

thxu for the quick reply...how long ruler to be used? as long as the fretboard?
n what do you mean by rock?sorry, i'm not very fluent in english...

:)
You'll either see a small gap between neck and ruler, at about the 7th fret (which is what you want to see)
so, this is normal?
thxu

what do you mean by rock?
Like a seesaw - the middle is higher than the ends.

You'll either see a small gap between neck and ruler, at about the 7th fret (which is what you want to see)
so, this is normal?
It's what is needed to allow the strings to vibrate. Take a piece of string, hold it at each end and pull it tight. You'll see the middle of the string vibrate up and down, whilst the ends remain steady - guitar strings do the same and you need to have a slightly bent neck to let them vibrate without banging on the frets (causing fret buzz).

Hi..wanna ask u this…my guitar has an angled headstock(like the one on Ibanez rg,pgm,etc)..got any advice (how or) where to put the guitar so I can sight the neck more accurately?

Btw, I've follow your advice but i don't 'have a good ruler for it..all are bent (following the fretboard) when I laid it on the fretboard so..instead using those ruler, can I do the instruction from u with the pencil box? it's more solid...(look at the attached picture)

On the attached picture.. I put my guitar body on my bed n the headstock flying,not touching any side of the bed ( is it good enough?) ….I 've done your advice with the pencil box,.i pressed one of my pencil box side (on the right of the picture) and I found a gap between my pencil box and my fretboard( look at the left side of the picture) n my diagnose is my guitar having a back bow..am I right?

Oh ya.i have done also the advice u've gave me....fretting the 1st fret and fretting (tap) the 14th fret…but the string is not making a straight line..it's making an up curve…not a big one though…is it good or is it a bad sign?
Thx u a lot…

If I've understood what you've done, I'd guess that you have a problem when fretting the lower frets.
If the string is curved, there can be no gap between string and frets - is that correct? If so, you need to release the truss rod slightly (i.e. turn anti-clockwise). Never turn more than 1/4 turn at one time and allow time for the neck to settle to it's new position.
If you have a capo, you can do this alone, if not, you'll need a friend to help you. Fret the first fret and the 14th - with a capo, you put it on the first fret and use one hand to fret the 14th (if no capo, you'll need the friend to help hold the string down or push the card in). Now take a visiting card (or card of similar thickness) and push it between the string and fret 7. You should have a good fit for the card. If the card is raising the string, the neck is too straight and needs another 1/4 turn (anti-clockwise). If the card slips under, without touching the string, you've gone too far and need to tighten the truss rod by a 1/8 turn. NEVER apply a lot of force to the truss rod, you may do some very expensive damage.
If you feel uncomfortable doing this, take it to a tech and let him do it.

another question,Greybeard..sorry..too much asking..i'm very confuse about it...i've just rethinking again with your post...
just fret the low E at the first and the 14th frets - the string will give you a straight line.

does the fret u have mention there is the one that we used to play natural harmonics?or fretting the string on the fingerboard?

which one should i see..the neck or the string?or should i compare the string n the neck?

btw, some of the articles i've read on the net told to tap the 7th fret also after fretting the 1st fret and the 14 th fret ..what is it for?and does the 'tap" word means to" press it down to the fingerboard" also? cause if it means to tap n press the 7th fret down to the fretboard too...., why is it neccessary to still put fretting the 12th fret?
(btw..still confuse which fret word meaning that u n those articles meant??)

n can i loosen my g string and d string?cause if not..it is so hard to turn the truss road because of the limied move...

btw..sorry for not telling u this from beggining...my guitar have a floyd rose system..is it have a different settings?

thx u so much...

does the fret u have mention there is the one that we used to play natural harmonics?or fretting the string on the fingerboard?
You need to press the string down, so that it is resting on the fretwires. The string tension will keep the string dead straight.
which one should i see..the neck or the string?or should i compare the string n the neck?
You are using the string as a straight-edge, to see whether there is a gap between the string and the 7th fret.
btw, some of the articles i've read on the net told to tap the 7th fret also after fretting the 1st fret and the 14 th fret ..what is it for?and does the 'tap" word means to" press it down to the fingerboard" also?
I can only assume the idea is that, if the string is not touching the 7th fret, there will be a sound, as you tap the string (caused by the string hitting the fretwire). If there is no sound, there is no gap.
All very well, but tapping won't tell you how far the string is, from the fret - you can only do that by looking and measuring.
n can i loosen my g string and d string?cause if not..it is so hard to turn the truss road because of the limied move...
You should really loosen all the strings - but if it is very hard to turn the truss rod adjuster, you should maybe consult a tech - a broken truss rod is very expensive to replace.
btw..sorry for not telling u this from beggining...my guitar have a floyd rose system..is it have a different settings?
As far as neck relief is concerned, it doesn't matter.

Hi, thx u for replying n not getting bored with me … :)

there is a gap between the string and the 7th fret

Actually, how far is the normal gap? Some article says 010" to .025", some says .003 to .006 inches..which one is true?

you should really loosen all the strings
…isn't the strings tension has an effect on the neck relief too?
I mean,when I tune the string..,doesn't the string tension will pull the neck too?

And If I loosen all the strings….How about the bridge ( as I have told you, my guitar has a floyd rose system bridge)? should I keep it parallel to the body? Thx u so much… :)