Take algae water in a glass or glass jar and let it stand for about an hour without moving. If green particles rise to the surface, it is blue-green algae.
The stick test
Test with a stick to see if the algae mass is solid and can be lifted. If the algae remains hanging on a stick, it is a harmless filamentous algae. Blue-green algae, on the other hand, decomposes on contact into particles in water.