Could not see "persist.service.adb.enable" value from default.prop of Android device?
According to the official android documentation, another way to get this value is to use the getprop command. (Link takes you to search results for getprop.)

I have not tested this.

I'm assuming you want to also set this value, not just get it? You can use the command line on the phone to do so.  (Disclaimer: I've only tried this in Donut, Froyo, and Gingerbread.)

setprop persist.service.adb.enable 1

This is the USB debugging setting; you can also just turn on USB debugging.