Offer the owner what you think it's worth, and if you don't have a deal immediately, give up on that one. Otherwise attempts to buy domains drag on interminably, which a startup can't afford.
Trying to buy domains is such a time-suck that it's usually less work to think of an untaken one.
You can look at various services to get a very rough guesstimate at how much traffic the domain gets...Alexa, Comscore, etc. You can also check the number of backlinks, etc. That's basically all the domainers do. Ultimately, you just have to judge what it is worth to you, though. It is rarely worth paying the quoted price.
Most "appraisals" from domain companies aren't worth the paper they're printed on. Everyone massively overestimates the value of 95% of domains...especially if they own the domain in question. Inflation of domain prices is so pervasive and out of control that it is almost humorous. There is a reason that most of the domains in the world are just 'parked' by squatters...MASS PSYCHOSIS.
Trying to buy domains is such a time-suck that it's usually less work to think of an untaken one.