These will, unfortunately, soon be a common finding in water sources in the upper U.S. where I live and is already being seen elsewhere in the warmer parts of the world.
You can tell the genus of the egg by its morphologic appearance. As pointed out by Tomáš Macháček, Anopheles eggs have lateral floats, Aedes don't lay eggs onto the water (but instead near the land/water interface, and singly rather than in floats) and Mansonia's eggs are laid in clusters just under the water surface (and have a prominent spiny ending).
Thanks to R Wendt who shared "I used to raise mosquitos and fed the females on my arms. They required a blood meal to produce eggs." Not a job I would want...
Here's a rather brutal but realistic poem from Blaine Mathison:
Found floating on the water of a neglected bird bath
was the beginning of a cycle that will cause such wrath
for soon skeeters will fly
and thousands may die!
As arboviruses spread along a devastating warpath!