The suggestion of watching the tank at night and observing for sweeper tentacles is a good one. As the corals get bigger and closer you will often see the sweepers during the day as well.
If you are looking for 'general guidelines' on the relative agressiveness of different types of coral then J Sprung has a book with a 1-10 rating for each coral listed. Sorry I don't own the book so I can't give you the title.
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