Bicycle Tutor

Picked up a used "Eska" bike...rides okay, but seems a little, well, a little off-balance. Can't tell if the frame is bent or not...if it is bent, is it possible to get that fixed, or is it better to just scrap it?

String method:-
http://www.sheldonbrown.com/frame-spacing.html
Also view the bike from the back or front and the wheels should be vertically parallel with each other.
Or / and - Without the mudguards ( if it has any) run a piece of string along the bike rims.
The string should just touch the rims, or tires, in 4 places if the wheels are in line.
Front and back of each rim / tire. You will have to do his below the bottom bracket line.

Hmm...the wheels do not line up, but the string test worked..seems straight from there...could it be the forks?

Difficult to tell?
All you can do is go round the bike and see if you can find the "fault",
Make sure the wheels are fitted properly in their dropouts.
Loosen the wheels and gently lean on the bike to settle the wheels while you re-tighten them.
Check the handlebars are not twisted or one side lower than the other.
The forks could be bent back slightly, which may not be obvious.
The headset could also be worn making the bike feel funny.