Things to look for (IN MY OPINION) in your spinning instructors:
1) does your spinning instructor teach off the bike as much as on the bike?
This means that the spinning instructor cares about your workout. Many instructors out there use their class time as their workout time, and IMO this is just not right. They can't possibly be fully aware of your safety while they are pushing 85% of their Max HR during a class.
2) what is your instructors experience on bike OUT of the spin class. Many instructors out there are fitness instructors, not cyclists. they do make for good instructors, but they don't fully understand the needs of roadies, mtb etc.
3) check their certification. different certifications teach the classes differently. spinning is spinning, reebok is reebok and etc. has the instructor kept up their certification and taken workshops? that means they are keeping up to date with their knowledge.
4) their personalities...some instructors are more spiritual types and some are hard core. Ask about how they teach, are they screamers push you hard types, or more encouraging calmly types.
5) they SHOULD ask you for some health background before you come in. also they SHOULD show you how to use the bike in a brief orientation before you start. if they don't bother, then find another spin studio to go to.
i hope this helps!
H