In Python 3 you can use str.isidentifier() to test whether a given string is a valid Python identifier/name.

>>> 'X'.isidentifier()
True
>>> 'X123'.isidentifier()
True
>>> '2'.isidentifier()
False
>>> 'while'.isidentifier()
True

The last example shows that you should also check whether the variable name clashes with a Python keyword:

>>> from keyword import iskeyword
>>> iskeyword('X')
False
>>> iskeyword('while')
True

So you could put that together in a function:

from keyword import iskeyword

def is_valid_variable_name(name):
    return name.isidentifier() and not iskeyword(name)

Another option, which works in Python 2 and 3, is to use the ast module:

from ast import parse

def is_valid_variable_name(name):
    try:
        parse('{} = None'.format(name))
        return True
    except SyntaxError, ValueError, TypeError:
        return False

>>> is_valid_variable_name('X')
True
>>> is_valid_variable_name('123')
False
>>> is_valid_variable_name('for')
False
>>> is_valid_variable_name('')
False
>>> is_valid_variable_name(42)
False

This will parse the assignment statement without actually executing it. It will pick up invalid identifiers as well as attempts to assign to a keyword. In the above code None is an arbitrary value to assign to the given name - it could be any valid expression for the RHS.