Using the below command is a quick way to pull Dell Service Tags quickly from a system without having to crawl around with a flash light.
Run this command in Command Prompt: wmic csproduct get vendor,name,identifyingnumber
This command will pull the Vendor, Name, and Service Tag
Note: I erased my Service Tag.
Additional way to get Service Tag:
kc provided an additional way to get the service tag with the command: wmic bios get serialnumber