I don't think you can, unless it is running very hot and the bass is quite obviously exagerated. If we're talking just a few dB's, maybe 3 - 5, depending on the sensitivity of your ears, it may be more difficult to tell. If you're listening to a recording where the bass has been already boosted, it might leave you thinking that the sub is running hot. Of course, the inverse would be true as well. If you're interested in accuracy, the only way to know for sure is with a SPL meter and test tones. Or, you could just leave it where it sounds good to you. In the end, that's what matters.
If there is a method you can use, which doesn't involve a SPL meter and test tones, I'm unaware of it.