Here’s a great post about transitivity.
The TLDR is
Transitive verb endings: す, せる, ぜる, める
Intransitive verb endings: ~ある, ~おる, れる
Often transitive: げる, てる, ねる, べる
Complete Guessing Game: ける, える, any other う ending
So most of the time e_u will be transitive and a_u will be intransitive. (Though, like any rule there are quite a few exceptions)