I jokingly told my manager "I quit" via email (it was longer than that) as an April Fools joke. We get along pretty well normally.
Well, it seems my joke was taken a bit too seriously as I just got an automated email from HR with checkout procedures and an invite to a few "checkout" meetings later this week as well. My company has also been letting a lot of people go recently so I am worried this is an excuse to have that happen to me.
I can see my manager responding in a like fashion but the HR communication worries me. He is also on vacation this week so I cannot ask him directly (which would be ideal).
How can I determine if I am actually being fired or my boss is just playing along?
Failing that, you should probably just quit your job - no one wants to work with a bunch of people who cannot take a joke. This is likely to make it hard for you to get a new job. Fortunately you can just make up your new resume. This will make is significantly easier to find a new job if they do not know to call your last one for a reference.
Call your boss immediately and indicate in no uncertain terms that the email was a joke (an incredibly bad idea, by the way), and you're concerned that it was taken seriously. The risk of being a sucker and falling for his automated HR email prank is much less disastrous than actually losing your job over a poorly thought out joke.
There's no need to take action. Just carry on doing your job as well as you can. If your boss is playing along with your "joke," nothing will happen.
Mind you, if at some point in the future your pay stops being processed, you can be certain that they have "fixed the glitch" and you actually were terminated. Please be advised that if you resigned in writing, you were not fired. Your prank is in much the same taste as having a lawyer deliver divorce papers on April 1st.