On a standard sectional one thumb knuckle length is about 10 miles. You can compare your own thumb knuckle to ten nautical miles against the radius of any VOR rose on a standard sectional, because they all have a 10 nm radius. So, using your thumb knuckle to measure the distance between a couple of points you pass on the map and on the ground, note the time it takes to cross those points, then put the ratio into the rotary side of the E6B. Where the 60 is on the bottom dial is your ground speed in knots.