Is it by intuition? Do I need a hazel rod? Because I'm buggered if I can spot it and we're only in a bog-standard 1930s semi, not some sprawling country estate. The mighty Anglian Water say they don't know either, and I've even found the original house deeds (we can't keep pigs or steam engines, apparently) and that doesn't say.
We have all sorts of random manholes but they all seem to lead to the sewer - I don't know if there's some way to tell if it's a manhole leading in or a manhole leading out (I'm sure there's some innuendo to be had there somewhere, but I'm too stressed by the dribble of water we have to work out what) water-wise.
We live in a 1950's semi ours is right near our front wall. Ours is on one side of what would be dividing fence and neighbours is on the other. Are you on a water meter if so stopcock is usually just behind it on your property. Sometimes they can be down quite deep in the ground ours now has a clay pipe around it and a little lift up flap. That was done by our builder when we had a cloakroom added on to the front.
I found mine (by looking for an apparently absent water meter!) under a little round manhole (that unsurprisingly said "water" on it ) in the middle of the public footpath right at the end of the drive........
I never did find the water meter - and the next door neighbour who's lived in her house for years says she doesn't know where hers is either
The stopcock should be under the ground at the front of the house usually under a small squareish cover. Sometimes in the front garden. Do you have an internal one under the kitchen sink perhaps. You can turn the water off with this but there should be an external one too. For the outside one you'll generally need a long metal tool to turn the tap because it'll be quite far down. We have had a house in the past where our stopcock and the neighbour's were both on our drive. Modern house so they shouldn't do that. Suppose it saved them money when building.