So i have finally found a sks at a decent price (for the rates of today :-/)
So basically how do you bench test it? How do I know its not trash? Any info is much appreciated

What exactly do you mean "bench test" it?

Do you mean shoot it and find out if it fires or not or what?

Have you bought it already? Do you have a bore light?

Familiarize your self with the operation of the rifle, check out the gas piston/tube, see how it looks.

I would be on the look out for rust (especially in the barrel and gas piston), general "bubba'd" repair jobs, cracks in the wood stock, signs of neglect, and also how clean the rifle is (if they are too lazy to clean the rifle pre-sale, how was the rifle normally treated?).

That is what I would look for. What country is the rifle from? There are some good guides on the differences and rarity out there.

Laying a gun on a bench and doing a function test on it to test if all the moving parts work so when you do shot it, you have better peace of mind that there is less chance for something to go wrong

Have you bought the rifle already?

If possible (before buying) I would check the gas piston and tube, the smaller rod the gas piston recoils into, remove the receiver cover, pull the recoil spring, pull the bolt. Inspect all of them for damage and wear, check the springs for breakage (they are coil springs), see if the firing pin freely floats, and is not stuck forward. Also check the inside of the receiver for cracks, damage etc.. Also check the harder steel cross bar aft of the magazine.

The primary safety concern I would have would be with the firing pin jammed forward it could fire fully automatically.

The trigger mechanism is very safe, same for the action, and the receiver is designed to be made of even mild steel, with just the cross bar having to be of a harder steel, so it is a quite safe design (as long as some one didn't muck with it, which I would look for).

If it is just an sks

I just quickly take the dude apart and look at all the parts if they look like they are okay I re-assemble and then try cycling the action. I check for rust, hung up firing pins, etc.