If you want to see how your child does against other students, then look at opportunities that compare him to other students. If he’s the last to be chosen on the sports teams, he is probably not a budding athlete. If he is often given the lead in the school play, he may have some acting/ public speaking talent.
Where is he placed in relation to other students? The advanced, average or slow group?
What are your and the other parent’s talents/ careers? Does your child show strengths in those areas?
What does your child do in his/ her spare time? I know two children who were fascinated with ancient Egypt and they both majored in international relations. One works at an NGO and the other wrote a history book.
Finally, why is this so important? Is it okay for your child to be average and happy? It is sometimes quite a burden we parents put on very talented children or they put on themselves. Truth be told, it’s not a bad life to go to the state university and major in something that earns you a solid, professional life and marry the person you met in sociology class and have or not have 2.3 kids, as you see fit. I have brilliant people in my family and they have invariably done good things with their lives and accomplished good things-- but they have often worked crazy hours, sacrificed personal happiness/ health/ sleep and sometimes relationships for achievement. If that is not what your child wants in life, then there is no reason to push it. Life will unfold.
If you want to know how to support your child, then I suggest listening to teachers/ looking at assignments in case there is a particular talent or deficit, and listening to your own child to support his/her interests. Regardless of these answers, do your best to encourage things your family values: participate in your religious community/ support their religious education, take historical trips, go to the library, take art classes, participate in a sport or take a morning jog with your child, volunteer, learn another language, teach your child to garden/ sew/ repair cars/ do construction. You never know when skills will come in handy or how those will develop.