Make sure it is CDMA Unlocked. Or a Sprint phone. If it is a Sprint phone - it MAY be possible for it to be Re-SKUed to work with Boost.
Unfortunately, there is no way to tell for certain unless you are able to attain the IMEI of the device. And even then - I've seen a phone that could work with Boost, but was still carrier locked and not able to work with Boost.
I activated a Sprint LG8 ThinQ on Boost. I had to have them send me a SIM card though, SIMOLW416TQ. After that, activation went smoothly. Definitely a great phone, worth the extra effort.