I was dx'd for DPDR last summer, but I suspect it's something I've struggled with for much longer.
For me, depersonalizing feels like my entire sensory experience shrinking down to one of my body parts. For instance, walking down the street and feeling like "a floating pair of eyes", laying in bed with depression and feeling like "just a pair of lungs breathing in and out", etc. etc.
Sometimes it's my heartbeat, other times my knee shuddering, but you get the idea. I describe it to other people like being under a glass dome with vaseline smeared on. Sounds are duller, my head feels cottony-- sometimes my mouth falls open and my tongue dries out. Usually I'm vaguely aware of my thoughts.
I can tell when an episode is starting to set in because my eyes unfocus and everything goes blurry.