the easiest way is probably to go to the Marketplace page of the given app and look there:
For more information you can also directly look at all of the app versions and see what version of the app is compatible with what version of Confluence.
BTW: Unless you're using apps that are not being maintained anymore, it's quite unlikely that one of your apps will be incompatible with Confluence 6.13, as it has been released in December 2018. :)
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