I have a problem with the brakes touches the pads. And just on the left. What's going on? How can I get rid of this?

 

A light rub is normal, but binding is not.

 

Never notice it before. But it sounds like its only on one side. How do I test if the disc are straight?

 

Use a dial indicator and check for lateral runout.

Or..... spin the wheel and listen for even drag.
A warped rotor will shake during braking.

 

My brakes developed a "wooden" feel to them. (Sorry but that's the only way I can describe it).
I lifted the front end and heard a definite binding area on my right hand disc whilst spinning the wheel, not so much that as it came to a stop it rested at the bind but it was there nonetheless. By eye it was very difficult to see and I employed a dry wipe marker to finally identify the warp.
I now have a post in the wanted section for my troubles. Best of luck with yours mate.

 

insert a screwdriver and pry the pads away from the disc slightly then bleed the brake

 

There is a specification for how much runout is acceptable. I agree that unacceptable runout is visually perceptible in a simple spin test (just need a friend to lift the wheel, or put up on stands).

Also agree that bleeding brakes is the best maintenance that a bike can have.