I always use a pitch pipe, small and inexpensive. However I always thought it was out of tune. This mystery has been resolved by my husband's new toy an expensive electric piano. Our 150 year old piano could not be tuned to concert pitch and I always took a note from that.
I cannot take a note or sing with a guitar as I hear many more harmonics than the average person. You might have the same problem. Good luck with the next gig.