I am not completely clear on your question…. are you asking about the internal ID of the Lead, the custom record, or custom record TYPE? Either way the URL is your key.
to find the record internal id regardless of type look for id= in the URL
If it’s the custom record you are looking for open one of the sub records in the UI and the same as above applies.
If its the Record TYPE you are looking for, with the sub record open look for look for rectype= in the URL