The Meadow Pipit is slimmer and has a longer tail. The Meadow Pipit doesn't have the characteristic minutes-long song of the Skylark.
Meadow Pipits and Tree Pipits only differ in some small details. Fortunately the songs are very different. The Meadow Pipit ends its song with a characteristic decrescendo tyu-tyu-tyu-tyu, while the Tree Pipit has a melodious song.