I put them on, and put a small piece of painters tape on the spot, and then take them off.
Then under a bright light, with a magnifying glass, I'll search the fabric on the opposite side of the tape, flexing and stretching the fabric in different ways, and using a light touch to feel for the "splinter" that's embedded in the fabric.
Once I find it, I pull it out with a tweezers.
Good long underwear are too expensive to throw away.
mmhmmm this is why we professionals never get caught in a hay loft (j/k). But good luck on getting hay out, I never figured it out. For every one you pull out there is 20 more in its place. You may think you got it all but its all still lurking waiting to poke your behind as you are driving 65 in the interstate
3m heavy duty scotch shipping/packaging tape. Unroll about 6 inches, flatten on cloth and pull up slowly from one end to the other. Repeat until it loses its stickiness.
It works very well on cat hairs on polartec, hay should be easier. Cat hairs can thread themselves through fabric more than once, its amazing.