I got a tag on my door informing me I have a possible leak. If I do it's not in the house. The ground isn't wet anywhere outside. How do I even know?

Make sure every fixture that uses water is off.  

Turn off valve to the toilets and make sure no one flushes them.  this is important later

Make sure icemaker is off, washing machine, dishwasher, etc.

Once you have verified everything is off go out to the water meter box and open it.

there should be some type of water usage indicator ( usually a spinning triangle)   & write down the meter reading.

watch the triangle for 5-10 min.  If it spins you have a leak between the meter & the house.

If it does not spin go inside and turn on the toilets listening to see if the toilet starts to fill up .

If it has not been flushed this could indicate you have a leaky toilet issue.  You can also but some dye in the tank to check for a leak.

Toilets are probably the #1 issue when it comes to a high water bill.

All that said, I had canton water stop my my house a while back and said I had a leak and she emailed me the logs of intermittent water usage.

After I did all of the above diagnosing, I did not have a leak.

It still should change if it's a digital?

All of the digital ones I've see have a visual meter on them as well, but that may not be universal.

City is going to send a tech out tomorrow

Have them give you some dye tablets for the toilets.

Funny- not ha ha.
In Woodstock, also have a leak... except I found it- hot water heater pressure relief valve just slightly hissing, tube was plumbed to the outside, so it was dribbling water "where it's supposed to"- for gawd knows how long. Having it worked on today. I'll post the plumber's contact info if it works out.

Looks like a small leak. The connector on my side of the meter. It's small enough to not turn the meter, but it fills the meter box with water. I'll get a plumber to fix it before it becomes a big leak.