There are a number of ways to test for this.
But first, don't Use any method that brings you in contact with the algae. The exception is, if you are working with a microbiologist on approved methods for algae detection.
The algae has a distinct color. So if it is present in large enough amounts, it can be seen visibly. And by playing with lighting and light detection methods, the sensitivity could be increased.
Water, whether lake water or stream water ,could be tested for phosphorus. There are test strips available, as well as EPA methods, that could be used.
This is a start. Hope it helps. Good luck.