Mold or dark-colored spots on the outside of the watermelon could indicate that it has gone bad.  The mold might be black, white, or green, and have a fuzzy appearance. The watermelon should have either a consistent pine green shade or a striped appearance.  Striped watermelons have alternating lime-green and darker pine-colored stripes. These colors indicate the watermelon is healthy.  If your watermelon is a different color (black, for instance), do not eat it. Different varieties of watermelon have different internal appearances.  Desert King, Tendergold, Yellow Baby, and Yellow Doll watermelons have yellow or orange flesh. When watermelon is no longer good, the crisp flesh will start to shrivel.  The flesh might even be pulling away from the seeds.  In other cases, the watermelon might become slimy and mushy. A healthy, edible watermelon should smell sweet and fresh.  If it smells tangy or sour, it’s gone bad and should be thrown it away.

How would you rephrase that briefly using Vietnamese?
Xem bề mặt vỏ có bị nổi mốc hay không. Xem vỏ dưa hấu có màu đẹp hay không. Xem bên trong dưa hấu có màu hồng đậm hoặc đỏ hay không. Lưu ý dưa hấu có phần thịt khô và xốp. Ngửi dưa trước khi cắt.