If you Google 'Kennedy Half Grading'.. You get a few websites with some idea of how to grade them. Here is one - https://coins.thefuntimesguide.com/how_to_grade_a_coin_grades/amp/
Before making a mistake, you have to determine if your Half Dollar is SMS or a business strike. They are graded on an entirely different level. Good luck!
So you're saying you're finding it difficult to find an absolutely free source of valuable information that can, if used in "weaponized" form, make one lots of money at the expense of others? Wow. Gee. I WONDER WHY THAT MIGHT BE?
If you are looking for examples of Kennedys in each grade, check here: https://www.pcgs.com/photograde/#/Kennedy/Grades However, in a more general sense, Kennedys are graded like every other coin, based on their strike, luster, eye appeal, and contact marks. For a much more detailed guide on how to grade coins, check out the book linked in my signature below.
