Here are some tips!
Talk to the band, ask as many questions as you can about the band and find out how well they can accommodate you, your event and your venue.
Try and watch them play if you can, if not, ask for video footage, whether this is professionally recorded or not, it will give you an insight to the band style and what they are like performing live.
Check out the bands website, is it informative, does it put you at ease and does it answer some of your concerns? Is it professional and does it give the right impression?
Whilst you have a list of questions about the band, think about what the band is asking you, is it reasonable?
Ultimately check out reviews about the band you are considering hiring, are they positive? Don't just go off the reviews on the bands website as these can easily be fictitious, check around and see if the band has any reviews from band agencies, band websites, or other directories such as FreeIndex.co.co.uk.