2000 Renault Megane Catalytic converter

Electrical problem
2000 Renault Megane Manual 86500 miles

My engine light came on (not the one with stop on it) whilst driving down motor way. Went to have it diagnosed and the chap said possible need new cat but needed to do more thorough check, so booked it to go in and took it back this morning. He had turned the light off and said possible it could come on again. Which it did after driving about 120 miles. Now he days it does need new cat, which I understand can be quite expensive. What troubles me most is there has been no difference in driving or any smell whatsoever. It drives as well as previous to the light coming on. Being a woman and not so good at the workings of a car dont want to be getting ripped off. Any advice you can give me or at least tell me what to look for to ensure it is the cat. It would be very much appreciated as Im a single parent and need my car for work but have limted savings available.