internal coolant leak...
1) losing coolant w/ no visible sign of where its going
2) white smoke at tail pipe
3) sweet smell at tailpipe
4) Coolant mixing with oil -- drain you oil.. capture the beginning of the stream into a clear container... and see how it looks, the coolant will separate onto the bottom.
5) pull a spark plug, look to see if it looks 'steam cleaned' compared to other ones
6) misfires on a particular cylinder
7) erratic idle
Another test I learned was to get a small piece of paper, roll it up, and stick it into the spark plug hole and pull it out and see if there is any coolant on it..
Almost all of those can be a sign of a leaking head gasket as well.... so other than a complete tear down, the only way I know to see as slipped liner is by pulling the oil pan... I've heard though, that sometimes a slipped liner is only visible when the engine is hot... so some mechanics get the engine warmed up, then drain the oil and drop the pan to see if it's slipped..
1966 Series IIa "Olive"
2001 Discovery II
2006 LR3 HSE7
Past: 1994 Defender 90 #416, 2001 Discovery II