The line between sport shirt and dress shirt is not always easy to draw, but look for the following sport-shirt hallmarks:
1. It feels baggy.
2. Its size tag says "Medium" rather than "15.5-34" (your neck and sleeve measurements).
3. When you button the top button and tie your tie, the collar looks weird. Sport (
4. The pattern is loud.
5. There are more than two colors, and they are not from the same family.
6. It does not have collar stays.
7. The fabric feels...more coarse and thicker than your dress shirts. (I know tautological advice is not helpful, but you will develop a feel for it with some experience.)
Sport shirts with a little blue in them are great because our denim culture means you'll often be wearing the shirt with jeans. Grab a blazer or jacket that complements the other colors, and you're set.
Some sport shirts may work as dress shirts if they fit you very well, don't look weird with a tie, and have collar stays. Something like this, if it met those criteria. Note the gray stripes. You probably could wear this shirt with a light gray suit, provided you find the perfect tie--one that picks up all or most of the stripes' colors without introducing any outre new ones (for example, red).
Sighted in the wild: loose-fit jeans; something close to this shirt, untucked, with cuff links; chocolate corduroy blazer; brown loafers.