The best way to tell is to do some research on the bike - there's plenty of information out on the web for most frames, and by far the majority of frames out there are only made out of one material (e.g. Litespeed Tuscany is always Ti, Trek 5200 is always CF). Don't listen to just the seller alone. There will probably also be a decal on the downtube telling you exactly what material it's made out of, and I'd think it would be pretty rare for someone to go to the trouble of faking this.

I'd also recommend you spend some time looking at different bikes in a bike shop. You'll get a good feel for identifying different frame materials.

In general, aluminum frames do have larger diameter tubes, steel frames have thinner tubes, and titanium frames are rarely fully painted leaving segments (or a whole frame worth) of brushed titanium finish. But, these are generalizations and there are plenty of exceptions out there.