If you don't have an insulated garage door, you're way behind. If you do, follow these simple instructions to evaluate the quality of your garage door insulation and see if there is more you can be doing to save energy and money. Remember, higher is better for R-values, because the higher the value, the more resistant the garage is to heat loss. The threshold for 'good' insulation is considered to be around 6.5 or so.
Professional tests used to evaluate the R-value are exceedingly complex and involve calculating standard deviations and z-scores and other measures of relative standing, and well as several-factor analysis of variance tests. However, you can use what's known as a quick 'heuristic', or rule of thumb, so make sure you're on the right track and your garage door is acceptable.
First of all, check with your manufacturer to see how much insulation came with your garage door. Then factor in if you've added some sort of insulating material since you got it. You then should perform a quick test- measure the temperature of your heated garage, turn off the heat on a cold day, and measure how long it takes for the air in the garage to drop five degrees. If this occurs within several hours, your R-value is too small.