How can I tell if my fridge freezer is leaking gas?

CFC's have not been used in fridges since the early 1990's.
It is unlikley you have a gas leak because all fridges have a set amount in them, if it leaks, loss of cooling will occur but it can not return as they do not refill themselves. (bit like a puncture in your tyre can not reinflate itself).
Your fault sounds more thermostat/motor etc.

Tony Randall fridge Engineer
January 2008