I started two from the ground up but didn't buy an existing business.
Make sure the numbers are accurate. Some people under-report income as far as the IRS is concerned, some people over-report so the business looks like it's worth more than it is.
Main thing you want to look for IMHO is cash flow and the income stream, i.e. how much income is there and how steady is it? You need to make enough to pay back on whatever loan you took to buy it and have some left over for yourself (unless you have another source of income to live on).
Look hard at the balance sheet, A/R, and A/P.
Do the employees look happy or are all the cars backed into the parking spaces? :lol:
As the new owner of a business, you may make some changes that the old employees are not happy with and some may leave. This can be a good thing or a bad thing. Unless a place is really screwed up, I'd advise making changes gradually.