To start with and be safe, you may want to put him in a small pipe chut that you can stick your head and hands through. Halter him and tie him with a very short lead. Present a mare to him if you have one and witness his reaction. Alternately you can rub your hand all over the mares pussy and let the stallion smell it. If no mare, just try to massage his penis. If his dick starts to grow some you may be in luck. At that point you can let him continue to drop before you touch him more or you can massage him as he drops to see his reaction. You may want to warm your hands up in the warm water before you touch him as they are pretty temperature sensatve down there. If he is not regularly cleaned, his penis will probably be one smelly mess. Bring some mineral oil with you or warm water in a bucket and wash his cock. Horses love warm water on their penis, often its enough to make him thrust on the spot. If your lucky enough with the warm water, and mare present, if you simultaneously put your lips over his glands and warm towel on his penis, he may shoot.
One thing for sure, you certainly do not want to be climbing under a stallion you have no clue about. That's a recipe for some serious injuries, and how you going to explain that. Stallions jump all over the place when excited and they can be all quiet and then suddenly they are jumping out of their hooves. Their is no controlling him at that point.
Good luck