How To Check If A Key in **kwargs Exists?

Python 3.2.3. There were some ideas listed here, which work on regular var's, but it seems **kwargs play by different rules... so why doesn't this work and how can I check to see if a key in **kwargs exists?

if kwargs['errormessage']:
    print("It exists")

I also think this should work, but it doesn't --

if errormessage in kwargs:
    print("yeah it's here")

I'm guessing because kwargs is iterable? Do I have to iterate through it just to check if a particular key is there?