How can I check the returned boolean of a function? I am attempting the check whether a function returns True or False in order to procede to the next step of a separate function. 

For example, here is a stripped down version of what I have:

def target(modifier):
    if modifier == 'MIRROR':
        return True
    elif modifier == 'ARRAY':
        return False

def execute(self, context):
    if target is True:
        print("True")
    else:
        print("False")


This does not work, though. How can I check what the return boolean of target() is?
The way you are doing it in the second block is not what you think you are doing.  The line 

if target is True:


will ask if a function reference is the same as a literal True...which it is not.  You want to invoke the function, so use the parenthesis and pass in the string you want.
If the parameter "context" is the one you are interested in comparing (if you are using BPY, it isn't, heads up), it will instead be:

if target(context):


Notice I drop the is True", if is doing that for you already, you don't need to do it again.