I just say to anyone wanting to instruct at our flying club "If the weather's OK when you come, we'll hopefully go for a quick trip. So just wear whatever you feel comfortable in. We're happy for our FIs to wear a club logo polo shirt and slacks - but if you feel happier in shirt and tie, that's fine - it's entirely your choice."
And the 'quick trip' is, of course, free of charge! Although the prospective FI will be asked to patter a few bits of various exercises, so that I can see whether he can fly and teach reasonably.