The easiest way to test an anvil or other potential “steel” object is to drop a ball bearing on it and see how high the rebound is.
Hardened steel should have a 70% or higher rebound rate, and usually higher.
If you don’t have a ball bearing, another piece of hardened steel, like a wrench could probably be used, by laying one end of the wrench on your opened palm, and allowing the opened end to drop side down on the anvil.
You not going to get the same rebound as a ball bearing, but you should get at least an inch or more.
A ball pien hammer should also work, dropped flat face down.
Any hardened steel, or even sort of hardened steel should rebound to an extent, and if it doesn’t, it should be avoided for an anvil.