Exactly the same way as you do to plumb the depth on a still water, or at least I do.
The other way is to set up your terminal tackle and then trot the float through the swim. If the float get's dragged under, move it shallow up, if it doesn't then go deeper. Eventually after about half a dozen trots (excuse the expression), you should be fishing either just off, on or slightly overdepth.
It's not quite as easy as on a still water, but it's still not that complicated.
More often than not if I fish a new stretch I will have a plumbing session all the way along before I ever fish it.
I use a large loafer float and an ounce bomb,cast it downstream and wait for the float to pass over the bomb,if it stays flat on the surface it's set too deep,if it goes under it's too shallow.
There is obviously only a brief time when the reading is correct because the flow will push the float under but it should be enough for you to see it alright.
If I already have a rough idea anyway I will sometimes substitute a bait dropper for the ounce bomb and kill two birds with one stone.
it's easiest of you've got a pole.
put 2oz lead on end of line ship out pole watch for point that elastic just goes back into pole set float to this depth job done.
thanks alot lads thats given me a couple of options,ill try them all and decide which is easiest,i fancied a go on the local stretch of the river irwell but that problem has always put me off
Although the advice above is sound I would offer an alternative especially if your river is relatively small.
Instead of overshotting with a plummet or extra shot, simply guess the depth and let the float slide through your swim. If it does not catch the bottom then increase the depth until it does.
This will be the accurate depth.
(Always bait up your hook though before the first glide through)
If your river is small (I don't know the Irwell at all) then all that splashing around will not do your chances of catching much good.
the stretch i am looking to fish is no more the 10 metres wide but"looks" fairly deep and it runs through at quite a pace with plenty of over hanging tree's