In Australia, clubs affiliated with the Australian Association of Live Steamers are covered by a common insurance policy. Any member of an AALS-affiliated club can therefore visit any other AALS-affiliated club with confidence that they are fully covered. AALS provides clubs with suitable membership cards for all members registered with the club, which through normal courtesy should be shown to any club visited to verify membership.
Many clubs also have some operating requirements which need to be attended to before you can run your own loco (safety briefing, signalling system orientation, club protocols etc). Like others have said, turning up on a public running day may not provide an opportunity to run your own loco unless you have previously been in contact. Some clubs will allow visiting locos from other clubs to assist in hauling passengers, some don't.
If in doubt, a phone call is the best idea.