Do not use another TDS meter to check if yours is working. Even if it's a brand new one and been factory set, it could still be out by a few PPM. Using the fluid is the best way as you know it is exactly 342 ppm.
Only thing that worries me is, if you use the fluid to test, would it not lose some of the salt in it, or maybe some of the water until eventually it is no longer 342 ppm?