How to Confirm Blue-Green Algae
A trained biologist can examine samples with a microscope to determine the exact type and number of organisms present, and some laboratories are equipped to test whether toxins from blue-green algae are present. However, anyone can perform a stick test or a jar test to determine whether a given algae sample is likely to be blue-green or another type. You can use this instruction page to perform jar and stick tests (PDF).
How to Evaluate My Risk if I Want to Get in the Water
While there is no hard and fast rule, a general guideline is: "the worse it looks, the worse the risk is likely to be." The greener the water, the more likely it is that it is dominated by blue-green algae, and the risk increases for both humans and animals. Situations to avoid include strong green or dark green water, the presence of observable floating chunks (algal colonies), or the presence of surface scum. In addition, if there is an offensive odor or stench, then contact with the water body should be avoided. Under these conditions, the risk of exposure to harmful algae increases. If a lake has signage posted warning of the presence of a HAB, it means experts have made an examination of that lake, and KDHE is obligated to inform the public of the risks of water contact.
For More Information
This United States Geological Survey publication (PDF) offers a much more in-depth look into the different strains of blue-green algae. When in doubt, stay out.