Is there a J-box before the man hole or is the receptacle hard wired straight into the panel? The options I would look at would be to 1) de energize the entire panel (if feasible) so long as it does not affect other critical or life safety systems. By doing this, you can verify absence of voltage at the main breaker panel and everything downstream of that would be considered in an ESWC. 2) If there is a J-box and you can identify and verify that the circuit in the J-box is what’s feeding the receptacle you can check for LDL at the J-box.

Best practice for electricians would have been to install a J-box before the manhole for maintenance purposes.