Hi Connor,
Unfortunately this is a big problem. The only official way is to have their bats tested. The machines that do this are very expensive and not readily available.
Some anecdotal ways that you can tell are that the ball hitting the bat will be louder and also from the smell coming from the rubber.
It is an unfortunate part of the sport at the moment. Until someone can come up with a cheap way of testing for these chemicals, I don't think this will be fixed.
Of course players that are using these if they get to a high enough level will have to change their habits. At major international tournaments there is racket testing that will catch them out.
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