Hydrostatic weighing would probably be the only way to tell for sure.
Calipers will be thrown off a little if you are bloated. You can judge by how fast you gained it. It takes time to put on noticeable fat, it won't happen in a few days. If you gain 10lbs in the 1st week of a cycle, it's water.
You could run an anti-e & see if you lose the water.