I followed the instructions at http://who-t.blogspot.de/2014/12/building-a-dpi-database-for-mice.html to measure the DPI of my good old USB mouse. The result is to be found below:

# Fujitsu Siemens
mouse:usb:v0461p4d16:name:USB Optical Mouse:
 MOUSE_DPI=500@125


$ udevadm info /dev/input/event2
P: /devices/pci0000:00/0000:00:12.0/usb6/6-5/6-5:1.0/0003:0461:4D16.0003/input/input2/event2
N: input/event2
S: input/by-id/usb-0461_USB_Optical_Mouse-event-mouse
S: input/by-path/pci-0000:00:12.0-usb-0:5:1.0-event-mouse
E: DEVLINKS=/dev/input/by-id/usb-0461_USB_Optical_Mouse-event-mouse /dev/input/by-path/pci-0000:00:12.0-usb-0:5:1.0-event-mouse
E: DEVNAME=/dev/input/event2
E: DEVPATH=/devices/pci0000:00/0000:00:12.0/usb6/6-5/6-5:1.0/0003:0461:4D16.0003/input/input2/event2
E: ID_BUS=usb
E: ID_INPUT=1
E: ID_INPUT_MOUSE=1
E: ID_MODEL=USB_Optical_Mouse
E: ID_MODEL_ENC=USB\x20Optical\x20Mouse
E: ID_MODEL_ID=4d16
E: ID_PATH=pci-0000:00:12.0-usb-0:5:1.0
E: ID_PATH_TAG=pci-0000_00_12_0-usb-0_5_1_0
E: ID_REVISION=0200
E: ID_SERIAL=0461_USB_Optical_Mouse
E: ID_TYPE=hid
E: ID_USB_DRIVER=usbhid
E: ID_USB_INTERFACES=:030102:
E: ID_USB_INTERFACE_NUM=00
E: ID_VENDOR=0461
E: ID_VENDOR_ENC=0461
E: ID_VENDOR_ID=0461
E: MAJOR=13
E: MINOR=66
E: MOUSE_DPI=500@125
E: SUBSYSTEM=input
E: USEC_INITIALIZED=955347