If you know a little about tubing, (Columbus and Reynolds tubing are desirable).

With very few exceptions- Trek's naming convention worked really well prior to 1986.

Very generally it was:
900 series= Columbus/Columbus
700 series= 531/531
600 series= 531/CrMo
500 series= CrMo/CrMo
400 series= CrMo or Manganese alloy/Hi-Ten
200 and 300 series= HiTen/HiTen

In 1986 the 400 series bikes came with a butted 531 frame with Trek (Tange) CrMo fork and stays. Essentially, that was what a 600 series bike had been the year before- and you'll notice there is no longer a 600 series bike in 1986.

The second number generally denotes the type of frame- Sport, Touring, Race. The early Treks were all touring geometry- in mid 78 they introduced the race frames- those were x3x. So a 730 would be a 531 framed race bike. Touring bikes had x1x. So a 510 would be a CrMo framed touring bike.

A little later 0 and 1 were used for sport bikes and 2 was used for touring bikes- the big exception was the 85/86 520- which was a sport/touring bike.

Somewhat later x60 and x70 denoted racing frames with the higher number meaning a more prestigious group set.