As suggested by others, Sandhills station is on the surface & all Northern Line services pass through there. Kirkdale depot is by the next station along the Ormskirk/Kirkby lines which usually has a few 507/508's loitering with intent to work!
For the Wirral Line, there's no one spot above ground where you can see all of the services. Either any station between Birkenhead Central to Hooton on the Chester/Ellesmere Port route, or Birkenhead Park and or North on the West Kirkby/New Brighton routes.
Some units are stabled at West Kirkby, New Brighton, Birkenhead North depot and Rock Ferry at certain times of the day too.