The LID is the radio ID number, programmed into the radio when all the talkgroups and system info is programmed in. This usually uniquely identifies the radio, so if someone hits their emergency button or something, dispatch knows who it was.
I think LID stands for "Logical IDentification", but maybe someone can correct me on this.
To determine what an LID is, most admins put them on the radio somewhere. To do it via RF, you'd need to either have an EDACS radio set up for decoding IDs, or to run a program like etrunk, which displays all system information.