If you are running Linux, the following command will show you the service tag:
root@bt:/etc# dmidecode -s system-serial-number
ABCDE1S
root@bt:/etc# dmidecode --type 1
# dmidecode 2.9
SMBIOS 2.4 present.
Handle 0x0100, DMI type 1, 27 bytes
System Information
Manufacturer: Dell Inc.
Product Name: Latitude D620
Version: Not Specified
Serial Number: ABCDE1S
Wake-up Type: Power Switch
SKU Number: Not Specified
Family:
If you are using Windows, use this command:
C:\>wmic bios get serialnumber
SerialNumber
ABCDEF1
C:\>wmic csproduct get vendor,name,identifyingnumber
IdentifyingNumber Name Vendor
ABCDEF1 PowerEdge 2950 Dell Inc.