Shall be grateful if someone could guide as what is the exact procedure for checking the oil level in my subject car gear box.

I have recently added some oil in the gear box on the instructions of independent oil change shop. Ever since i feel that the 3rd gear now have starting jumping to 4th gear with a jurk, which was not there before.

I personally feel that the oil quantity in the gear box have exceeded more then the limits or may be the quality of the oil is different from the one which was previously inside the gear box.

Kindly advise first what is the right procedure of checking the oil level and what should i do so that the gear box is not damaged with these un necessary jurks.