Onion services will use your relay to store descriptors once it has the
HSDir flag.
https://metrics.torproject.org/rs.html#details/ED279FC87D17921131CB629BA1D4E6B0F8C00BF8)

Is the HSDir flag the same as V2Dir, or something different? You correctly identified my relay. I see it is a V2Dir but do not know how to determine if it is a HSDir.
You can't determine which onion services or how many onion
services are using you without code changes.

Doing this is considered being a malicious relay and will get your relay
removed from the Tor network.
If this question came up and was sent to tor-dev@ because you intend to
perform research: it may be prudent to stop what you're doing and
content the Research Safety Board.

https://research.torproject.org/safetyboard/
I'm just a curious user, this is not part of a formal research project.  Is there a public tor testnet that I could have joined my node to for this sort of experimentation (I don't have the resources to build my own)? I can understand why you don't want production nodes logging data that could potentially cause harm to other network users, but also do not think curious users should be discouraged from poking around in the code.