Usually you count the # of pedal rotations for a short time period and multiply to see how many revolutions per minute you are cycling at. Examples:

count for 10 seconds and multiply by 6. (10 seconds x 6 = 60 seconds or 1 minute)
count for 15 seconds and multiply by 4 (15 seconds x 4 = 60 seconds or 1 minute)
count for 20 seconds and multiply by 3 .... and so on...

The longer you count for (and lower you multiply by) the more accurate your number will be.

-OR-

You can just ride for fun and not worry about top speed. If someone asks me how fast I can ride- I lie. Its a lot easier than doing math and I can get way more street cred for a lot less effort.