Phil Howard, who works for awesome maker firm Pimoroni, has a pet project he and others (including @RogueHal13) have been working on for a couple of years now: pinout.xyz. It is a fantastic database of add-on boards for the Raspberry Pi that shows you what GPIO pins are being used, what for, and which ones are, therefore, free. So, for instance, this is the link to the SenseHAT page which shows you that the only pins used are the I2C  pins. Here’s a picture of the interface, which you can see is really well done.

It really is an excellent site. Phil even accepts GitHub submissions to the site to keep it up-to-date with new boards as they are released. So, if you’re ever in need of finding out that vital piece of information about an add-on board, head over to pinout.xyz.