--- Is the piano in tune? (I can recommend a tuner)
--- Is there electricity if an electric piano is used?
--- Do you have cues for the beginnings of pieces?
--- Is there light to see the music?
--- If there is a soloist, a rehearsal with the pianist is recommended.
--- Is there a clear written understanding of what music will be played?
--- Is the payment plan clear?
http://www.weddingvendors.com/planning/articles/tips-for-choosing-wedding-musicians/