What Mike said - put *something* in the monitors to start with, and hope it's OK to get the band through until you get round to doing a better job of it. But fixing monitors is secondary to getting it OK at FOH.
If you have the misfortune to mix our band you will soon know if the monitors are wrong - we'll just stop playing until it is right. It just so happened that for our first few gigs we had some great monitor engineers who had the knack of getting things right straight away so now our lead singer expects every sound engineer to live up to that standard. Oddly enough - all the good monitor engineers we know trained by the same PA guy in Portsmouth.
JRP Music - Audio Mastering and Restoration.