One good method is to look at the ads and see if they're actually products you do/would buy or use. In Departures (Amex Platinum members) you get $50K watches and Masaratis. In Afar you get ads for destinations and offbeat tours. In between you've got National Geographic Traveler, T&L, and Conde Nast. In my opinion the latter two are interchangeable. Afar and NGT are the most respected by travel writers---as in they have the best writing/writers who win a lot of awards each year.
I write for Global Traveler, so I won't plug that one
