There are several ways you can do it. I never found the steel bar worked very well. Sometimes it did, but you got too many false echoes and look a right prat when you made them dig in the wrong place.
These days, you can use ground penetrating radar - but even that has its problems ... false signals, and bloody expensive. We even put radio transmitters in the pipes, dragged them through and tracked the signal with above-ground receivers. We also pioneered the use of miniature TVs, dragging them through the pipes and watching on a monitor. With modern technology, you can also use GPS tracking, in combination with the cameras.
The old fashioned ways, though, still work. The most obvious way is to ask the owner. There were times when you got the keen youngster, beavering away, and you'd spot somebody watching him through the window. You'd go and speak to them and they told you what you wanted to know. You'd ask why they didn't tell "the lad". 'Cos 'e didn't ask! would be the response.
The second - and very often most successful way - was to find the "little old lady". In a street, there would always be one ... been there for ever. You had to be patient, but if you kept with it, they knew everything. Ah yes, she would say, that was done by Bert's cousin, good lad he was ... it was just after Ethel 'ad her kid, and there he was ...
Sometimes, they came up to you on a job and asked what you were doing. If you were officious, they just buggered off without saying anything but if you gave them the time of day, and got chatting, they'd suddenly say, "you know there's a drain there ...? somewhere you hadn't dreamed of looking.
One of them, amazingly, had complete plans. Her dad, long dead, had been the builder and she'd kept them as a keepsake. She let me sit in her parlour and trace out the entire drainage system. Good job that - spent the rest of the day in the pub and still got a pat on the back from the boss for doing it so fast.
But, failing all that, you wait until it snows. In a fresh fall, the snow melts over the covers. You just go round, mark them off and Bob's yer uncle (or builder).
We are a satellite state of the Greater European Empire, ruled by a supreme government in Brussels. We owe this government neither loyalty nor obedience. It is not our government. It is theirs. It is our enemy.