Hardly scientific, but I test block heaters/battery warmers for basic function (i.e. does it draw power?) at work by plugging it into the outlet in the handle of my trouble light. If the trouble light dims slightly when I plug it in, the component is using current. The assumption is - as much as I hate assumptions - that if it's visibly drawing power and didn't trip the breaker on my light, it's probably working properly. I honestly wouldn't know offhand what a "good" resistance rating for a block heater was unless it tested as obviously bad as being shorted or excessively high.
A word to the wise - before plugging one of these things in for the first time in a season, do a quick visual on the cord first, for as much of its length as possible. I can't count how many cords I've seen with exposed conductors from abrasion or damage from hot or moving engine parts. That could really suck.
