SOME SELLERS UNKNOWINGLY BIN COUNTERFIT BATTERIES. IF YOU SEE ANY RED FLAGS, IT MAY BE COUNTERFIT.

If you buy a used camera (Mirrorless, DSLR, P&S), any included battery is likely near EOL. Batteries are considered "consumable" with limited coverage. This is especially true with a used camera.

There is no catch-all method to check this. Many older cameras lack support for this, or it is restricted to prosumer and pro cameras. On cameras with poor monitoring, you will need to make a judgment call. See this 80D as an example of proper health monitoring.

Refer to your camera's menu system to see how to check this as each brand is different. For example, the battery I got with my Canon 80D from 6/2017 is essentially in very poor condition and "end-of-life". I still use it but I do not trust it in the field without a healthy pair of spares.

The seller this 80D came from "included" this battery, but likely knew it was nearing the end of it's usable life. I cannot verify this as absolute fact, but this is VERY common with used cameras.

Batteries without monitoring: If your battery has no/poor monitoring, look for a date stamp. If it is over 3 years old WITH limited runtime, consider it consumed.