If booking with EI direct you can find your fare class before purchasing described in this thread
Fare Class Before Booking
If you've already booked it you can find your fare class in the confirmation email (it will have something like S/Economy Class where S is the fare class). You can use that info then to check if you will earn avios with your BA number included (not all fare classes earn Avios with BA) on the BA website avios calculator (you will earn no tier points). Or for avios open an aerclub account (using same details as BAEC) and you can combine your avios with your BA account for free later on. Check earning rate for United using fare class and up to you what you deem more valuable. For United earning there are lots of reports of miles not posting or taking a long time to do so.