I've noticed the best indicator of my busyness is my email inbox.
When I'm not busy, my inbox has less than 5 emails in it - often zero. I'm easily able to keep up without any problems. Having an empty inbox is a good feeling. A REALLY good feeling. So when I'm not busy an empty inbox is what I strive for. This period lasts for about 3 weeks.
When I get busy, my inbox fills up fast. I actually start to realize just how much email I go through in a day. I currently have 40 emails, and it will probably climb to over 60 or 70 before I start whittling it down again. Which if I'm falling past behavior, will be in a few days because that'll be end of this busy week.
Then the cycle will complete and within a couple days I'll be back down to an empty inbox. It's crazy - I know.
I don't know if I have some deeply insightful point - I wish I did - I just thought I'd share what I've noticed. I guess that by acknowledging this pattern it helps me cope. As my inbox climbs closer to the century mark, I think it starts to bug me. It starts influencing my choices and I start opting to take care of other things in life. I suppose that's a good thing.
I now know that my inbox is filling up because of a choice I'm making. I can now decide to slow down and catch up, or decide to wait and no matter what it'll be OK because I know I can, and will, easily catch back up. So that's good too.
Here's a point: So, if you're sending me an email and I don't reply right away, fear not. I'm just busy and will definitely get back to you later. About a week later.