New horse in my horses paddock? hoping she doesn't get pregnant?

You don't need to get close to the horse to tell, you just have to look between his back legs. Even at a distance you can't really miss them. They are, for lack of more polite wording, really big balls. Also if he is a stallion you can always request an abortion shot from your vet. My friend, who owns a stallion (and likely shouldn't, but it isn't my problem) keeps him Ina pasture. However mares escape regularly to see him, so if it is a mare who she does not want pregnant she is given an abortion shot. Which also led to an amusing story of a very stupid intern giving the shot to her gelding. Thankfully that one turned out okay, but if he had been a stud it would have rendered him sterile.