How do you "diagnose" separation anxiety?

This evening my husband and I pull into our apartment complex after a night out.  We'd driven separately, and I get out of my car while he's still maneuvering his car into the parking space.  I walk up the steps to our apartment door and hear a dog barking.  Surely it isn't my dog, right?  But I stop and listen, and after about a minute of silence I hear it again.  A dog--most definitely Mona--barking and carrying on like crazy for 15-20 seconds.  It sounds loud, frantic, and almost pained or scared.  Another 30 seconds or so of silence.  Then the barking, then silence.  By this point my husband walks up and listens too.  She barks again, and once she's been quiet for a bit we unlock the door and walk in.

I've been coming up with all the excuses I can think of: she got excited when she heard our cars pull in; she saw our headlights in the window; the cat had done something to antagonize her; she'd already spent a lot of time in her crate that day and had just had enough.  And maybe one of those is true.  But... I'm not so sure.  I recall one time, shortly after we adopted her, hearing her cry after I'd locked the door behind me.  It seemed to stop after a minute or two, so I left.  But what if it didn't stop?  What if she becomes so stressed while we're away that she carries on like this every time we leave her?  Has she been suffering from terrible separation anxiety this entire time and we never knew it?  The thought of it makes me feel terrible for her.  Not to mention her barking is loud, and I hate to think the neighbors have been suffering through it this whole time.

We have always crated Mona any time we are away.  We're working on leaving her out for short periods of time, but she likes to chew too much to be trusted out by herself for more than half an hour or so.  Our schedules vary daily, but I'd say she averages about 6 hours of crate time a day, five days a week (she is also crated at bedtime).  Usually, if we're going to be gone for over 6 hours, I am able to come let her out to potty at lunch. Today she was crated from about 8:30 to 4.  That's really the longest she's ever crated during the day.  I never thought it was an issue... She's never had any accidents in her crate, and she never appears stressed when I come home.  Well, I've never heard that barking before, anyway.

So, this barking has me really concerned... Are my concerns justified?  Or is it perfectly possible that this was an isolated incident, and she's usually just fine?  I suppose the best thing to do would be to stick around the apartment for a bit before I actually leave and listen in on her.  Likewise, when I come home after being gone, I should probably hang around outside the door before I unlock it and listen in then too.  And hopefully I won't hear anything.

...But if I do, what's the next step?  Ugh.