How do you tell if a automatic hyundai excel gbox is slipping

If you step on the gas and the engine accelerates but the car doesn't then that's slippage.

If you press the brake at a stop while you step on the gas you can determine the stall speed of your torque converter, if the engine revs beyond the stall speed then it's the torque converter clutch, TCC clutch solenoid, or a bad converter. Otherwise you need a professionals help.