I work for a very large construction company. When they need to dig in areas where they are not certain of the undergrounds, they "pot-hole" first. A commercial potholer is a trailer mounted rig with a water lance and a vacuum rig. While I haven't tried it at home (yet) a pressure washer and a wet-dry vacuum are conceptually the same.
It's messy work, Tyvek suits and face shields required, but will not damage underground utilities. Usually done by two guys, one with the lance and the other with the vacuum hose.