Re: How to address a Shihan, Sempai, or Sensei?
If you don't know, ask.
If you address someone with a black belt as "Sensei" and they get incredibly incensed, you might want to find a new dojo.
The use of "Sempai" varies from school to school, and it is bestowed either as a title or an assumptive title anywhere from nikyu to nidan.
"Shihan" tends to be a more exclusive title, but as Bob said: call someone however they introduce themselves to you, and it's always better to be overpolite than underpolite. Often, karate systems will show the practitioner's rank (and sometimes their title) on their belts: since most aikido systems just have belt/hakama, it can be hard to distinguish exactly who is what, so when in doubt, ask!