Call a plumber, of course.

Unfortunately you're not going to be able to buy a magic gizmo on Amazon to help with this. There are leak detection services that use specialized equipment to locate a leak and you may have to resort to this. It will cost a couple hundred dollars. The problem is that if it's below a paved driveway, even the pros may have difficulty.

woodbutcher-

Good point, though I would think they would have noticed a leak inside by now. Unfortunately, if the leak is outside and under the driveway it's going to be a pain to find. The only other thought I had was whether there is an irrigation system in the yard. I had a major leak a few years ago and after some digging found an irrigation supply line with a badly glued PVC pipe joint that apparently took a number of years to fail. If there is an irrigation system, then it, too, should have a separate shutoff valve which would help isolate the problem.

It took until the carpet in our basement family room was wet (stepping on it, was like you were wringing a sponge) to realize that we had a problem that required a plumber. Turned out that the water going down the drains in the showers, the bathroom sinks, the kitchen sink too, and probably from every toilet flush - had been coming up and out of the laundry standpipe because there was a pipe blockage from a pair of child's socks that were somehow flushed down the toilet in the bathroom that our kids use. It was quite a mess, and though our homeowners insurance covered the remediation and restoration process (after payment of the premium and the initial cost to have the plumber come out and unblock the pipe), it was a hassle, took about 4 months from start to finish, and ultimately raised the insurance cost by a few hundred dollars when it came time to renew the policy.

The guys who were doing the restoration part of things told me that they've had jobs where a leak wasn't noticed for YEARS due to where it originated, and I was lucky to have caught mine so early. Sometimes, people just don't notice that their roof has been compromised, or there's a pipe in the wall somewhere that has slowly but consistently been leaking for a long time and they only found out when they had it inspected while trying to sell or things got to a point where the leak caused catastrophic (and obvious) issues.

Eliminate the potential indoor problems first, then work your way outwards.

MsTina- I wish you good luck with getting this resolved. Please report back what the specific problem was when you get it fixed.

Outside and under the driveway is a main supply from your city leak that gushes. Or a slow sewage leak that seeps. Gushing erodes soil, runs your meter up big time, and is easier to find. Seepage would use a camera put in at the exterior clean out where the main line leaves the building. Either needs the pro.

So my hubby and his nephew are heading to the rental property today. He did have a plumber come out
a few weeks ago. Not sure what the plumber did, but both of them determined it’s under the driveway. Hubby tried to run a line and it hits something he hits something. Plumber thinks it’s been compacted down by dirt due to the driveway weight and that’s prob why we’re having an issue now. But it is a 1920’s house so most likely no work has been done for a very long time. So now the driveway gets torn up even more today. I’ll follow up to our findings.

Exterior supply leak would not show in a bill in my area as the meter is in the house. My parent's house has the water meter in a vault by the street so it would be noticeable on their usage figures. I agree the quantity of water would saturate the ground and be easier to find than a sewer leak.

One of the local plumbers that specializes in drains here does a cheap camera job as a way of getting leads for their very pricey pipe lining projects. If someone in your area has a similar deal, you could find more information before committing to a huge fee. Either way, this will be expensive

Are we talking about a water supply line that is leaking or a drain pipe that's clogged?

From the original post it sounded like a supply line. But the follow up states that hubby 'tried to run a line and it hits something'. So that sounds like putting something down the drain pipe.

greg-

I agree. I think, based of the use of the term "leak," that many of us made the assumption this was a water supply leak, since leak is not a term typically applied to sewer line problems. A good reminder to us to get an OP to clarify, before spending time giving advice on the wrong problem... ;-)