If it’s an Opuntiad, i.e. in the Opuntioideae subfamily of Cactaceae, i.e. generally in the genus of Opuntia, Cylindropuntia, or Austrocylindropuntia, plus a few other genuses like Consolea which look like Opuntias, and Quiabentia which are really cool, then you:
1. Wait for it to flower. Take a picture, it will last longer.
2. Check out this website Opuntiads Web.
3.Click one of these links to get to the flower pictures:
4. Match up the pictures and grab the ID.
5. Send me the photo and the proposed ID and I will be happy to concur.