What eactly do you meanand how can I check that?

At this point you could remove the front wheels, secure the rotors to
their hubs with the lug nuts (torque them to specs if possible), put a
dial indicator on the rotors and check for axial runout.  Then you
could mic the thickness of both rotors around several different spots
and note the maximum-minimum variation per rotor.  If axial runout is >
approx .002" then either the hub or the rotor is not true (probably the
rotor, dontcha think?)  If thickness variation is much > .001" that can
tend to cause a pulsating brake pedal and front shake when braking.
These numbers aren't set in stone, they're just generic readings where
a vibration or pulsation can begin to occur.

Let's say you find that the rotors weren't turned properly... My advice
IF you have a K1500 (you just wrote 1996 Chevy 4x4) is to buy new
rotors; I've found it difficult to keep those rotors true after a
couple turns.  They range in cost from about $25 to $50 each depending
on brand.  If you have an S-10/Blazer 4x4, their rotors are fairly
inexpensive as well.  I have less experience with them, but have not
heard anything about a high rate of rotor warpage in those models.

Toyota MDT in MO