How to determine if a string is escaped unicode

How do you determine if a string contains escaped unicode so you know whether or not to run .decode("unicode-escape")?

For example:

test.py

# -*- coding: utf-8 -*-
str_escaped = '"A\u0026B"'
str_unicode = '"Война́ и миръ"'

arr_all_strings = [str_escaped, str_unicode]

def is_escaped_unicode(str):
    #how do I determine if this is escaped unicode?
    pass

for str in arr_all_strings:
    if is_escaped_unicode(str):
        str = str.decode("unicode-escape")
    print str

Current output:

"A\u0026B"
"Война́ и миръ"

Expected output:

"A&B"
"Война́ и миръ"

How do I define is_escaped_unicode(str) to determine if the string that's passed is actually escaped unicode?