You never know. I had a mare I obtained when she was 26. She had been broke at age 3 and not ridden since. I put a bridle on her and slid on bareback in her pipe corral, just to see how accepting she would be, and she went right along with the program. Next time I came out to see her I brought a light saddle and rode her a bit in the arena. In the end, though we rarely rode her, she was like riding a green broke 26 year old horse. Very gentle and quiet, but did not know much. I was the main one that rode her whenever, and had some really fun times with her, but was always careful not to over do it. My favorite memory of her was riding her by myself on the sand next to the lake and going for a long canter. I rarely rode her past a walk, but she had the most lovely gaits. She was a very special horse and lived to be 35.
I would do the same with these guys. If they are respectful on the ground, try them in a very controlled enviornment, and if you are new to horses, I would have a more experienced rider work with them and find out what they do and don't know. They may be very well trained, or, like my old mare, may just have the basics. A gpod trainer will help you assess them.