I cannot for some reason tell whether a guitar neck is straight or not by holding it vertically up. How am I suppose to look so I can tell.

I'm also wondering is there a technique to tell whether your frets are straight or not?

Hold it vertically and view it from the side (i.e. the part that would be the 'top' where the tiny fret reminders sometimes are, not the actual front of the fretboard). If you want to be more thorough, remove the strings and see if you can place the flat parts of the fretboard (i.e. the entire 'face') along something you know to be straight such as a decent wall or sideboard etc...

As for strings, do the same, but focus on the bridge area. I've had a bastard of a problem with the G string being too low relative to the other strings, but going to try sort it when i re string and clean my axe

Ohh it says frets, may as well leave the string bit. I wouldn't worry too much about this, if they were significantly out of shape you'd clearly know about it. You could view each one individually, by looking at it or using some apparatus such as a set square and ruler to check levels and balance etc. This depends on how stoned or bored you are as to how thorough your examination is

If i understand you right....You want to check how much your neck is bent? Meaning the curvature(if thats how you spell it) of it?

The way i have read to do it is....

1- With your fretting hand hold down the low E string on the 1st fret.
2- with your pick hand (thumb) press down on the same string on the fret where the neck and the body meet (somwhere from 13-15)
3 - Then stretch your index finger on the picking hand (while still holding the thumb down) to about 7th or 9th fret. The distance between the string and the fret board is how much your neck is bent....if there is no space then your neck is bent in the wrong direction....I have read that there should be just enough space to slide a playing card in between the string and fretboard.

I hope you understand what i meant....Its kinda hard to explain without showing you.