A nine-letter word with but one vowel,
After I’m used you may need a towel,
Tests can be used to learn what I am,
Once I am known you can hit a grand slam.
Strengths. It has 9 letters with only one vowel, when you use your strengths the exertion can cause you to sweat, requiring a towel, tests can identify ones strengths and once you know what they are, you can hit a metaphorical grand slam by employing them in an area of your choosing.