How can I tell a good piper from a "marginal" player?

Once you've got a contact, and assuming that you have some time, don't just ask if they have an instrument and a kilt.  Ask about his/her musical experience.

• Ask how long they've been playing: It takes about three years of study before a piper should be doing ANY solo gigs - even for friends. Most players begin to reach their limits after about seven or eight years, but every player reaches a different development plateau. (read about The Development of a Piper.)
• Ask about their performance experience:  Have they done your type of event previously?  What do they recommend for your event?  Can the play the tunes you want?
• Ask about competition experience:  Most assertive players, working hard to get ahead, will have competed.  (read What do the grade levels mean?) (Note: I think the worst I've ever heard of was a person who'd never taken a lesson, much less competed in a band above Grade IV, nor as a soloist, calling themselves a "Master Piper".)
• Ask about their learning experiences:  Active tutelage (giving and/or receiving) or attendance at a summer piping school is a good sign of musicianship.
• Ask if they are playing with a band:  Current or past experience with local pipe bands or folk groups also a good sign of musicianship.
• Ask for a demo tape:  If they're serious about playing, (s)he'll be able to provide you with a demo tape with tunes suitable for your event. If you don't like the tape, don't do it. Find another piper.
• Develop your piping "ear".