MK14 sort of mentioned it above.
- Set multimeter to 10A DC range.
- Connect the '+' red lead of your multimeter to the 12V supply positive, connect the '-' black lead to the "12V" terminal on the LED strip.
- Connect the 'R' on LED strip to the ground of your 12V supply (black).
- The LEDs for red should light, and the multimeter will give you a current reading. Can repeat for the other color channels, testing one at a time.
Hopefully you have a spare 12V supply (3A+ rated), or old PC power supply to use for testing. Otherwise you'd have to use your main PCs 12V supply..
https://www.electronicshub.org/current-measurement-using-multimeter/