Some manufacturers have bike catalog archives you can look at, and the specifications should indicate what kind of bb a bike has. I know Cannondale, Trek, and Jamis have archives online, and others are likely too.

If you're buying second hand off eBay or elsewhere, you can just always ask the seller for frame specs. I personally prefer threaded bb shell for ease of use, but as mentioned, I'd use the Praxis adapter if I liked the frame but wanted to stick with threaded external bearing bb.