Press the string against the first and last frets and look around the 12th fret--is there space between the string and the 12th fret? (If your fingers are long enough, you can do a tactile check by tapping the string against the 12th while fretting the first and last.) If there'a a little space, you're fine, unless there's a lot of space, in which case you'd need to actually tighten the truss rod. If there's zero space, if the string is touching the 12th fret, then you need to loosen the truss rod a bit.
If all that checks out, it could be a bad fret higher up.
Btw, there's a setup subsection on these forums. Don't worry, I've posted in the "wrong" places here, too. lol