Pure O: How do I tell the difference between obsessions and compulsions?

There's been discussions lately on the OCD support lists about how to tell the difference between an obsession and a compulsion, if it's all thoughts, as in "Pure O." My understanding is that the initial thought is the obsession, and the cascade of thoughts afterwards are the compulsion. For me it would be something like, "What if I said the wrong thing?" as the obsession, followed by trying to figure out if I did indeed say the wrong thing, including retracing my words, trying to account for all of them, which is the compulsion.

The irony is that the compulsion is supposed to be what reduces the anxiety produced by the obsessive thought, and yet, when I used to say "I wish I could stop obsessing about this" what I really meant was the whole flood of compulsions. I fly into compulsing so quickly that it seemed quite dubious that the initial thought was causing the anxiety--surely all the retracing, figuring out, analyzing, and research were the obsession right? How could they possibly be a way to lower my anxiety??

But what I learned during exposure therapy was that compulsions only provide short term relief, and in some cases, very very short term, and then they rebound with their own additional suffering, taking up mental space and energy, and that if I challenged myself to refrain from figuring out an obsessive thought, that my anxiety level immediately spiked, and that was a sign that the compulsion was serving its function of a short term hit of relief, even if I couldn't see it as it happened.

My husband had the flu this week, and he gave this analogy--the virus is the obsession, and the immune system response is the compulsion. When we are sick, what makes us feel lousy isn't the virus itself, but the attack of the immune system on the virus. In the case of our bodies, for the flu or other illnesses, we actually want the immune system to attack, but OCD is more like an allergy, where our immune system attacks something harmless like pollen, mistaking it for an invader. I know the things we obsess about don't seem harmless, and that they are often about things important to us, but our full blown compulsions cause us more misery in many cases than the initial obsession.

In the midst of my worst OCD flare ups, I had glimpses of how destructive the compulsions were, but I was so scared by the obsessive thought that I clung to my compulsions. I finally hit a low point with my health anxiety, that even though I was terrified of getting treatment, I knew I couldn't continue on the way I was going and have any kind of life. If you are like me, you probably also spend time trying to figure out if something is an obsession or a compulsion, and wanting to know for sure which it is. Another lovely complication of the OCD! Take your best guess.