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.
