Whatever fit you pick, check these:

• Shoulder seam: For regular, it should sit right at the edge of your shoulder. For oversized, it can drop 1–2 cm, but not more.
• Side seams: A proper tee has side seams that align front and back. If they twist, the tee was sewn poorly.
• Length: Regular should hit mid-hip. Oversized can be longer, but if it goes past your crotch, it's a dress or a tall size.
• Sleeve cuff: Should hug your arm without squeezing. Oversized sleeves should be loose but not flapping.
• Fabric weight: As mentioned, medium weight is best. Avoid thin, see-through fabric unless that's the look.

These are easy to check when you try on or even when you buy online if the site lists measurements.