The SWR meter that is on the front of the radio is very accurate and can be trusted.
You do not need to use an external SWR meter with these radios. If you feel that the readings on the front of the radio aren't accurate, you can certainly use an external meter to verify the measurements are the same.
When setting your SWR, you might need to turn the TSR function off (located in the Menu). With it on, it will stop the radio from transmitting with a high SWR. While usually a good thing, it makes it difficult to set your SWR.
After you complete the tuning process, we recommend turning the function back on.