I'm doing A level chemistry at the moment, for revising subjects do a bunch of exam questions or practice questions, and if you're struggling to answer them you probably know you'll need to revise that part of the subject.
It also may be a good idea to use something like quizlet to memorise some key terms too.
For the maths side of chemistry (Moles) you'll definitely wanna practice some exam questions on them
Also, practice some questions which ask you about experiments