Here's a nice graphic I got from the Traeger® Grilling Guide.
Color and temperature are key. The redder is it, the more rare the steak is. Here are the ideal temperatures for each degree of doneness:
Rare - 120 to 130 degrees F
Medium Rare - 130 to 135 degrees F
Medium - 140 to 145 degrees F
Medium Well - 155 to 160 degrees F
Well - 170+ degrees F