How to prove wallet ownership?
How can I verify that an arbitrary transaction contains a
human-readable signature?

There is no such thing as &quot;human-readable signature&quot;. The signature is just a very big number. Or we can treat it as a pair of very big numbers. Usually we encode it in hex, so it looks like 3044022007ae5e36b79303f992e153798061458b435d34285c5fddc20655451ac71b49f6022079421d162cad51db0daa0a934cbf5261d4b53f185604e9790094a49145f3812f01

Which information of the transaction data should I use to obtain a
human readable form of the transaction signature?

Input scripts

Which online resources could be used to verify a signature?

Google for &quot;ecdsa verify signature online&quot;. But I am not sure that these resources are suitable to verify bitcoin signatures.