On the B744 freighter used to allow enough time to check to ensure that I was happy. Would check that all the locks and latches were correct and all loads securely fastened as in turbulence you dont want any items coming loose and puncturing the cabin. Also check for smells and spills.....not so obvious with livestock!
As for weight well you have to trust the loadmaster has the correct figures from the shippers.
Remember in God we trust....all else we check!