here are a couple of rules of thumb (based on your question, and the premises: )

Avoid any bike that:

- you cannot lift and walk for a couple miles on your shoulder
- has tubular tires (based on the OPs premises)
- has visible rust on the chain and the drivetrain components (that would most likely be the rear derailleur)
- has broken teeth
- has bangs (or holes) on the frame (paint missing is fine, but actual bangs and dents will affect the structural integrity - i.e. the frame might "break")

Things to consider:

Ride the thing in a parking lot (preferably big and empty) looking down at your front wheel:

Hit that front brake a couple of times. If the wheel wobbles or you hear noises, you'll need a repair/parts.
Focus on the perimeter (edges) of the front tire while riding. If it looks like it's moving on anything else than a straight line, you will need a repair/adjustment/parts

Flip the bike and have it rest upside down on the seat and handlebars (don't do that if it is a pristine condition $1000+ italian steel bike that would make the seller get of the roof, with good reason):

spin the rear wheel and look at the same things as the front wheel (mentioned above)

-feel the bike... if it speaks to you, get it.