Probably the best plan is to fly above any fair weather cumulus. Less bumpy, fewer gliders.
Just wanted to repeat that cause I think it's the most important advice here, other than avoiding a gliding site altogether.
In a glider it's very hard to climb in any kind of thermal above the first layer of fair weather cumulus. You'd have to go IMC (in the cumulus itself), or be launched (either by tug or winch) above it. So by flying above the first cumulus-layer, you are pretty safe from gliding activity. Not entirely, but I dare say 99% of the gliding activity will be below that first layer.
Exception of course is when there's ridge or wave lift. Particularly in wave fantastic heights can be achieved.