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.
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.