If you're really concerned that the chain might be long, it's easy enough to confirm either way.

Shift to small/small, and wind the RD cage clockwise. If you can do so enough to pull up the slack, then the chain is short enough. If you can't pull out the slack before hitting the RD's cage stop, then it's a bit long.

In any case, shortening the chain carries it's own risk, which is far more serious than a slight long chain. Most mechanics measure chains by the big/big+1 method, which yields the shortest safe length. So, shift to big/big and check how much chain you have to spare before cutting.

In any case, you just paid a professional to do a job. If you're uncomfortable or concerned with the results you should bring it back and have him check his work and explain how and why it's correct. Part of what you pay a pro for t=is the peace of mind that it's done right. If you don't have that, you're letting yourself be shortchanged.