first boats (and 2nd boats in big colleges) are "seat raced". This is where the trialling rowers competing for a place in these high up boats go out in two boats. Either 2 fours, or 2 eights depending on the number of people trialling. Surplus stand on the side of the river when not being raced. You then race the two boats over a set distance (not v long cos you get tired doing it over and over) and see who wins. Then they swap some people around and race them again and see who wins then. Then they can work out who is the better rowers.
It's also done on 2k erg times, and general commitment/keenness