I tend to do both, but most things I have an idea of the points before. I buy minced turkey rather than beef( hate quorn) the treat bits I know the points of already. I did but double dockers the other day but dissapointed when found out they were 8 pp each. Now they are sit the the cupboard calling me!
With meat I weigh it up before cooking to work out the points, try to stay away from red meat though.
Hope this helps xx