The front end are the best way to distinguish the generations from each other:

The "series" (=prior to 1985) have a not-aligned front end (picture left) and are based on a 88" or 109" wheelbase, on leaf springs. The aligned radiator grill / front lights are built after 1985 and have a 90" , 110" or 130" wheelbase. Still in production today, they use coil springs.

 

The Series all have the grill not aligned with front end/ lights. Above are the three Series One (1948-1960) versions. On the left, a rare exemplar pre-production from 1948, date of birth of Land Rover. One year later, the front lights go through the grill and it is now possible to drive at night ! On the right picture, the most common Series 1, from 1952.

 

The next generation is the Series II and IIA from 1961 to 1971 (left) with mechanical improvments compared to the Series 1. On the right picture, a typical Series Three, with the plastic grill and headlamps on the sides, not in the centre panel anymore, due to new regulations.

 

The main difficulty for the rooky is to distinguish the Series One from the Series Two (above pictures). The One have sides that are not rounded and less high. The rounding of the front wings is wider. On the right picture, the Series Two, with higher wings and shorter rounding.

For more details and better explanations, visit specialised websites like

http://www.lr-mad.co.uk/history.html