What absolutsnwbrdr said.
Here's a link to a bolt circle template you can download and print to check the bolt circle on the pop-up.
http://www.easternmarine.com/em_store/t ... ircle.html
A great majority of pop-ups came with Dexter axles (either tube or torsion type) so parts are pretty easy to find. If you think you'll need brakes, Dexter sells brake kits for almost every axle they've made, so that's a plus if it doesn't already have brakes on it. Look for an aluminum tag wrapped around the axle tube - that'll have the axle part number and serial number on it, which comes in VERY handy when looking for bearings and seals.