i think the best way to really tell is to do an x-ray before the bones of the BP are dissolved. it needs to be early enough in the digestion process and detailed enough to show the thin bones of snakes.
apart from that, the only things that survive digestion are some hair and some parts of the teeth. reptiles dont have hair, which leaves the teeth. but figuring it out will be groce and nasty, basically you need to collect all the poop, dissolve it, and work it through a fine sieve and sort until you got teeth, and then identify the teeth.
causing the boa to regurgitate / throw up... well, its an option, i dont know if its a good idea. it wont help the BP and might hurt the boa.
an x-ray to check for a BP skeleton or BP skeleton remnants is probarbly the most elegant solution.
of course the fact that a large boa got into the BP enclosure means that the small BP had a good opportunity to get out of the enclosure.
could there be a chance that someone pranked you and just swapped the snakes, did you check the boa enclosure for a small hiding BP? if you have no clue how the boa escaped its own enclosure and got into the BP enclosure, that might indicate human involvement.
This is not a prank because 99% of the time, I'm the only one that goes into my snake room. This is just a terrible time because i have my final exams on monday and tuesday...