Ask questions if possible as to what his issues are, if hes lethargic or sensative. Etc. When I get on, I start with giving the horse the benefit of the doubt and start light and soft you will know in a few minutes what he needs.
If I have to ride a new horse I just do what I would do with a familiar horse, because I know I'm the boss, throughout the ride you should become more comfortable with the horse. :)
Ride as many horses as you can get your hands on! Always challenge yourself although this doesn't mean taking up riding a horse that is way out of your skill level. Also, start small and then work your way up with cues. You don't want to put too much leg on a horse that requires a soft hand and seat etc
Relax figure out the horse's habits and quirks to make your ride