I was visiting Quonochontuag Beach, Rhode Island over the holiday weekend. While I was there, I took advantage of the sand to check on my running technique.
If you leave a clean footprint in the sand when running, it's a good indication that your technique is sound. Here is an example of my footprint (below in the center). This is, more or less, what you want your footprint to look like when running in the sand.
Here is an example (below in the center) from my wife who is not a runner. It is not a clean foot print, and it is clearly showing that she was attempting to push herself forward as she ran.
If you don't get a clean foot print when running in the sand, it doesn't necessarily mean that your technique, on other surfaces, is bad. Many people change their gate when they run in sand, especially if it is an unfamiliar surface. However, you can use this technique as a training tool. Because, if you can learn to run on the sand in a way that leaves clean foot prints behind, your technique, in general, is likely to improve.