Will Marine Test Kit work for Fresh Water?

I think you'll find that the chemicals are the same, but the colors produced are different. If your kit is anything like my API kits, saltwater produces a shade or two darker than freshwater. So the presence of ammonia tests green, but in freshwater it's a clear green instead of a cloudy green in saltwater.

I thought about doing the same thing when I first got a saltwater tank, but decided it wasn't worth the risk.

For my Hagen nitrate test kit, the saltwater readings are higher at lesser concentrations of color. The best way and probably only reliable way to figure it out would be to calibrate your kit by creating a known solution of whatever you are testing and then test to see what color that solution comes up as.