"Open Mike" should mean just that, unless there's an obvious clue like the club being named "The Traditional Irish Instrumental Music ONLY Club." The rub comes in finding out what level of proficiency will be tolerated. Our local club (NH, US) allows just about anything, and those of us with standards just smile tightly and bear it. Heck, musicians have to start somewhere. A problem would arise if you've got microphone hoggers with no talent, or people who claim free admission as a "performer" because they read a couple of half-assed poems or tell some jokes they read on Mudcat.