So, how do you know that the ground beef is fully cooked when you are stir frying it on a sauce pan ?
For the color blind people:
If you're cooking the meat in crumbled form (not burgers), after you've broken it up and let it cook for a couple of minutes, add a cup of water to the pan.
Continue cooking until the water has evaporated. You might have some fat left in the pan if it was a fattier grind, but the moisture will both help prevent overcooking the beef, and give another indicator of when it's cooked long enough.
The USDA explicitly warns NOT to rely on color as an indication of doneness. Per the USDA, meat may brown before it is safe to eat and also meat may be safe to eat before it browns.
If your recipe has you cook the meat after it is browned (e.g., brown the meat, then add ingredients, then cook in an oven for 25 minutes), you're probably fine. If you're using browned meat as-is, check that the internal temperature is 160°F using a cooking thermometer.
When you buy ground beef you see it has white spots. That is what makes it oily. You will know when it's fully cooked when it is a medium brown color it should be ready. Make sure to keep an eyes on it when it's dry add a bit of water. Then cover it. Then you do this once more let it dry and it should be ready. MAKE SURE THERE IS NO RED OR PINK