If you don't have a GPS or other speedometer, you can make a speedometer with a string attached to a tennis ball.
What you do is toss the ball overboard and when it crosses the transom, start a timer.
When the string is fully extended (pulled taught), you stop timing and figure out your speed.
By my quick calculation, if you had 100 (actually 101.2) feet of rope, the following would be accurate: