Incidentally, Mode C is not the same as altitude reporting.
This can cause endless confusion when reading specifications or trying to understand control panels etc.
An aircraft without an altitude encoder will still reply to Mode C interrogations, it just doesn't include any data inside the pulse frames.
In otherwords, XL319, the question I think you are trying to ask is whether or not the aircraft has altitude encoding, rather than does it have an "A+C" setting on its control head.
Been a while since I've flow a light aircraft, but if I recall correctly they are usually placarded to say there is an altitude encoder. If there is no placard, you might be wise to assume there is no encoder.
The owner / operator should be able to tell you. If you're the owner, check with an avionic engineer.
pb