To determine fit:
* Try to watch them work in their current job for at least a day or two. Not meetings, not just talking with their top guy, but sit with the average person you might work with, talk with them, and understand what is going on.
* Get a thorough understanding of the process you would have to use everyday in your job. You probably will have to do things differently. The way they do things is what you can expect a year or two in.
* This isn't relevant for acquihiring but if you were to have a product or service that would be integrated, estimate the maximum time you would be doing integration work and not actually able to work on your product. Set your expectation to that and hopefully you won't be let down later when you have no time to work on or support your own product.
In the end, if you aren't excited about working for their company, don't work for them. The money isn't worth your sanity or soul.