Moped Army

So i was riding today, stopped at a light .. idled a bit, then i heard a CLUNK CLUNK CLUNK when i tried to take off then all i did was rev the engine up reall high, backtire didnt move ..

at first, i thought it was the chain .. checked the chain, not the problem the back tire still moves from the chain moving, but not when i go to throttle it. .. now im thinkin its the trans because ive had the clunkin noise before but I just ignored it .. (big mistake, broke down far away from home)

any ideas on how to troubleshoot it? how can I check to see if the trans did slip, or put it back in place?

thanks to all in advance