They way I did it was just doing a linepack speed test. I had downloaded it for free from the market before installing a rom with an o/c kernal. Speed tests would vary for everyone but I was getting around 4.5 mflops before using setcpu to change speeds at 800mhz I was getting around 6.8 mflops. Using gbhil's method will tell you for sure though, I just know that we are not all comfortable typing commands.
You're right, and sometimes I tend to forget that
I'm sure other file managers will do this, but here's how you can check with Astro:
Open the app and navigate through the files to:
/sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freq
Long press and choose open as: text. The file only has one line, and it's a number. That's the top end of your CPU. If it says 550000 you're stock. Higher than that, you're overclocked. Lower, you're underclocked.
Be sure NOT to save any changes when you close the text viewer down. If you do by mistake, reboot your phone to correct it.