A fit name is a starting point. Garment measurements tell you more about how a shirt may sit on your body. Treat regular, slim and oversized as a direction, not a size recommendation.
Colour may catch your eye first, but the neckline is something you feel throughout the day. It affects the room around your neck and how the shirt looks underneath an open layer.
Bring your blazer when trying on a T-shirt. A shirt can fit well on its own yet bunch at the shoulders or hang too low underneath a blazer. Checking both together makes the choice clearer.
A soft fabric feels good in your hand. Whether you will enjoy wearing the shirt becomes clearer when you try it on. Take a moment to check the neckline, seams and room to move.
Pick up two T-shirts and one may feel lighter than the other. You might see 220 or 330 GSM in the description. That number is useful, but it only tells you part of the story.