You could always ask "OK...what do bands typically take home in tips from your club?" If the club owner doesn't want to say or seems like he's lying (giving a ridiculously high figure, etc), you can follow up with "Can you tell me which other bands have played for you here so I can check with them? I need to know, so we can be sure we will cover gas and other expenses our band will be incurring." If he's unwilling to do either of those things, don't play there.
You could also see if you could take home a percentage of the cover charges, if there are any...though that doesn't always work out well in the band's favor, either.
Guaranteeing a certain number of people that will be showing up is a good tactic...if you draw less, you don't get paid...but make sure at least that many people come...and make sure they spend money too, so the club/bar will have you back in the future.