You don't need waterfalls, beaches, or volcanos to test what exposure length your camera can handle without excessive noise, ISO settings, tripod issues, sunlight versus cloudy, etc. You can test all that in your back yard.

The only thing you can't test that way is optimal length of exposure for the effect you are trying to achieve. However, that is easily done on the spot. Simply try a few different exposure lengths, within the range you have previously determined your camera can handle. Most of these scenes work well from a few seconds to maybe 30 seconds, which is often the limit where digital cameras start introducing artifacts. Perhaps you can get away with 60 seconds. Beyond that, you either need to combine multiple exposures with a short shutter close time in between them, or use film.

Since the exposure times will be relatively short, you can simply try a range and see what you get by looking at the preview. Actually, I'd take a large range anyway, since the look of the picture will be different for different exposure times, and you may like having the different views of the scene later.

That said, a few years ago I happened to have my camera and a tripod with me as I came across a small cascade. I didn't have a ND filter, so my longest exposure was limited by ISO and f-stop. I took a few pictures just to have a record of the effect over a small range of exposure times.

This is 1/2 second:

1 second:

4 seconds:

In case anyone cares, this was 7 Oct 2011 in October Mountain State Park in western Massachusetts.