Recommendations from other golfers is also important.  They may provide you with recommendations or you may get those comments personally from other golfers.

But of course the real test of a golf instructor is whether or not your swing and your game actually improves.  It will take time – few see an improvement after just one or two lessons – but you should see a better swing with more control and distance soon enough.  If you go a few months and still are not improving, you may need to find another instructor!