Standard methods are:
- Ask for a standardized exam. Examples are the GCSE A-levels and SATs for undergraduate admissions, and the GRE or GMAT for graduate admissions.
- Ask for a percentile in addition to the grade itself. A student who is 99th percentile is better than 99% of students in his/her cohort. It's not perfect since it's possible that the cohort is broadly better at one university than the other, but it's still a strong sign that the student is good.
- Calibrate based on prior experience. If an A-student from university X attends this program and does well, then future A-students from that university are more likely to do well. With thousands of universities in the world however, this is not really possible for every university.
- Say "screw it" and rely on letters of recommendation, and Google for the letter-writer's credentials.
Incidentally the problem is difficult enough that there are verification services for academic credentials. Example. I've never used one of these though and don't know how useful they actually are.