one word... Hydrophone
my way of finding them, this is mostly in 1939 and early 1940 draw lines from the Norwegian port of Bergen to Aberdeen, Firth of Forth, Hartlepool and Inverness, then pick one of those lines, find a position within 50km of that line and within 100km of the British coastline then loiter until some unsuspecting merchie comes along, this shouldn't take more than a day or 2, Identify nationality (very important) and sink if hostile....
don't get too near the coastline though because of minefields. I also find the deep areas NE of Hartlepool and E of Rosyth good spots for hunting with a good depth for getting away from any nasties that show up