You only need three things:
a. A bike with fenders (well, duh).
b. A cyclist with hairy legs.
c. A rainy day.
The test protocol is as follows:
1. Allow c to go on long enough to produce puddles on the road.
2. Install b on a.
3. Instruct b to go for a ride.
4. At the conclusion of said ride, inspect the leg hair of b.
(hey, nobody said science was pretty)
Is it just wet? That's just rain from above. Fender passes inspection.
Is it wet and (ee-yew) gritty? That's road schmutz that got past the fender. Fail.
(Testing rear fenders involves the inspection of different anatomy, but as this is a family blog, I'll just leave it at that.)