How to select a good male sheep at about 7 months age?

You don't need a ram. They can get really dangerous. Plus, you don't. You deal with a reputable breeder who can, genetically, select the proper one for the quality of your ewes. If you are just raising meat lambs, anything will impregnate your ewes. If you epect to sell breeding stock you need a registered ram to get quality offspring, allowing that your ewes are quality themselves.