Rust on chassis - surface is OK but holes will need fixing. Rust on bulkhead, footwells, lower brackets, top and window hinges.
Noisy gearbox (they're never quiet but shouldn't be too whiney or clunky).
Make sure low range and high range fwd engages and disengages.
Noisy and/or smoky engine.
Excessive play in steering - a good one will probably have at least an inch of play at the steering wheel rim.
Balls should be shiny.
Not too many serious oil leaks - most places weep slightly.
All electrics working.
Brakes aren't great but shouldn't pull to either side.
Hand brake should hold on a reasonable slope.
Should have the right tyres - SWB are often fitted with bigger tyres which can improve the gearing but buggers the speedo reading.
VIN number on plate should be correct - the chassis number will probably be missing or unreadable due to corrosion or repair. (on right front chassis leg)
Does it look right? Landys are often cobbled together with bits from other vehicles but they should be the right vehicles. If it's made up of bits from other models you might find sourcing parts a bit of a nightmare.
Later engines are often fitted - make sure you know which one.
Cosmetically, older landys can look a bit rough. It doesn't matter!
Here's some background reading :-
pdf Land Rover Manuals | Landroverweb.com