I think doing a good job of kicking the tires and really understanding the business before you decide to give up the paycheck is critical. Figure out what you can do while still employed before giving up the paycheck. Talk to lots of folks in the business. Find out how they got started, what they'd do differently if they started over, what they found surprising, if they would mentor you, etc.
One other way I've used to collect information quickly is to create an online survey on the business to find out information like revenues, costs, licensing, marketing ideas, time to ramp up to current volumes, etc. I identified a number of businesses like the one I was thinking of starting and asked them to participate in a bench marking study and gave a free copy of the results back to participants. My most recent survey got a 20% response rate. People like getting the data back. You just have to be professional and treat their information with confidentiality. It helped me find out who was the best and I followed up with a call to find out more.
Yardstick Golf Indoor Golf. Measured for Success.