Just time how a train takes to travel a measured distance. Here's an example:
Measure off a distance of 100 (actual) inches, and measure the time it takes the train to cover that distance - say it takes 50 sec.
That means at the same speed it would travel 100 * 60/50 inches in a minute, and 100 * 60*60/50 = 7200 inches in an hour.
Then convert the 7200 inches to your scale, for N scale that would be 7200 * 160 = 1,152,200 inches per hour, which is about 18.2 miles per hour (1,152,000 / 12 inches per foot / 5280 feet per mile).
Converting all this to a magic formula:
N-Scale MPH = Distance traveled in real inches /
(0.11 * Seconds taken to travel that distance)