On the Northern Line, units are stabled at Kirkdale depot and Southport. There are usually five sets at Southport on weekdays which are used in peak times, and normally five or more at Kirkdale. On winter weekends, there are more units at both locations with fewer trains required in service.
On the Wirral Line, most units are kept at Birkenhead North depot, with one normally at Birkenhead Central after morning peak. It is rare to see anything stabled at Rock Ferry on Mondays - Saturdays, not sure about Sundays. You'll also usually find units at New Brighton and West Kirby out of use on weekends, but rarely weekdays.
In terms of sighting units, you could easily see all of the Northern Line units in service at Liverpool Central or Sandhills stations. If you're not hanging around for peak hour, you might as well hop on an Ormskirk or Kirkby train to Kirkdale and back to see what's in the depot there.
As for the Wirral Line, if you don't fancy the short staircase between platforms at Hamilton Square, you could always just hang around Liverpool Central and see them there.
There are 36 units used off-peak (18 on both lines) with 12 added for the evening peak period. The Southport route uses 10 units, Ormskirk 5, Kirkby 3, New Brighton 4, West Kirby 5, Chester 6 and Ellesmere Port 3.
I'd prioritise the Northern Line, where you should be able to catch a few stabled units at Kirkdale. It's a lot more difficult at Birkenhead North where sets are often kept within the shed until just before the evening rush.
Just out of interest, which units do you require? One or two are out of service just now for overhauls and re-livery work.