what is the correct way to check for False?

It rather depends on what somevalue can be: if somevalue could be anything you could check that it's a boolean and not:

if isinstance(somevalue, bool) and not somevalue

this doesn't rely on False being a singleton. If it always is a singleton you can also do:

if somevalue is False

But PEP8 of Python states you shouldn't care if it about the class and just use:

if not somevalue

this will evaluate if somevalue is "falsy". See Python documentation on Truth value testing.

PEP8 states:

Don't compare boolean values to True or False using == .

and gives these examples:

Yes:   if greeting:
No:    if greeting == True:
Worse: if greeting is True:

which translates in your case to:

Yes:   if not greeting:
No:    if greeting == False:
Worse: if greeting is False:

Keep in mind that each string is considered "truthy" except the empty string ''.