Standard stuff Mike:
1) Does it start?
2) Rust
3) Is it missing anything?
4) Does it run?
5) Does it have parts that don't belong on it?
6) Check around the headstock for bulging. If there is bulging, it was re-aligned.
7) Check under the seat pan for rust
8 ) Check for alignment
9) Ownership papers?
10) Check eBay for parts ... see how available parts are
11) check every part of the frame where something bolts on for rust and wear
12) Rims, spokes, hub - check'em. Replacing wheels is a pain in the ass and can get expensive.
13) Check the alternator. I don't know if replacements or upgrades are available. The replacement for my BMW cost me nearly $600
according to this month's Vitage Motorcycle price guide, here's what you should be paying for a '75 ducati 500 gtl
6) $620 - Basket Case
5) $930 - Almost complete restorable - not running
4) $1,400 - Good - Ridable & complete
3) $1,860 - Very good - Partial restoration, complete
2) $2,480 - Fine - Well restored
1) $3,100 - Excellent - Show bike