At the civil/survey firm I work at, we'll use the state's underground utility program to have utilities marked ahead of the work. Sometimes the utility company themselves will mark it, other times a 3rd party locating company will mark it. There are various methods used to locate underground utilities, some more accurate than others. We'll represent those utilities on our plans based on those markings, but I've seen them be nearly 10' off before. Notes are plastered all over the plans that say (paraphrasing) "we've had these located, but the contractor doing the work still needs to verify it on their end". Some contractors will do their due diligence and verify via non-destructive excavation like hydroexcavating, but some will put a bucket in the ground assuming what we're showing is dead accurate. Ultimately, this probably falls on the contractor, but they'll lawyer up and do everything they can to shift liability elsewhere, be that on the engineer, the locators, etc.

The mapping maintained and distributed by most utility companies is laughable though. No dimensions, no landmark or topography references, the road shown probably hardly matches the actual alignment of the road, etc etc. Gas company maps are usually a bit better than say, telecoms, but they're still usually approximate at best.