I have a 03 coupe with Luff M1, 19x8.5 front, and 19x10.5 rear wheels. Using Hankook k104 245/35/19 front, and 275/30/19 rear.

Dropped it with Tein S-techs, 1.2"front, 0.9"rear.

How often should I balance the wheels under normal driving conditions?

How would I know if the wheels need balancing, what signs?

How would I know if the wheels are bent?

 

never had a bent rim, but i believe there is a LOT of road noise and you can hear that something is wrong if its bent... someone correct me if im wrong

 

Well my problem is that the steering wheel shakes after passing 75 MPH. Before 75mph everything is cool. But at 75, the steering shakes. Its not a bad bad shake, but its one that for sure you would notice.

I have already gotten the front wheels replaced and balanced. Problem is still there.

What could it be?

I am running hankook 245/35/19 and 275/30/19 tires, and the car is lowered with Tein S-techs. But I don't think its a spring or tire problem since the shake doesn't happen at all speeds but only over 75MPH.

 

Assuming the balance was good, take it to either a good suspension shop or the dealer and have them check the outer tie rod ends. Further, there might be a defective ball joint elswhere in the system.
Most shops will charge little or nothing at all to do a front end check as this is how they make money. One other possible problem is the rotors. If the brakes are warped (happens, esp with hard use ), a vibration will result as described.