10-15 minutes of walking with you on him? Leading? being sponged off? At a CTR you will sponge your horse will an entire bucket full of water until the entire bucket worth is on their body before a pulse is taken. A lot of horses don't acheive their "normal" pulse after anyway...but that's what you aim for. Also, make sure the horse is not fidgeting or eating when you try to take the pulse, that will make it seem higher than it is.
Anyway, there are a lot of ways to go about assessing fitness. Part of that is the Pulse and Respiration after a ride. You don't want their respiration higher than their pulse, that is "inverted" and is not a good sign. Mainly, you want that pulse number low...40 is an excellent place to be after a CTR :) 60, not so much.
Aside from that, learning to check your horse for dehydration will also clue you into their fitness. One way is the skin pinch test, just pinch their skin and count the seconds before it goes back to normal(you want it to go back quickly...if it's taking 2 seconds your horse needs water) and along the same lines you can press into the gums and see when the white spot goes away. Among all these harder ways, don't forget the simple things like how tired they act or how much they sweat!
As far as fitness translating from arena to trail, you do need to consider the fact that there will be hills involved, therefore it will be tougher at first if your horse isn't used to that. I would just start out with a distance and speed that you KNOW he can do and that won't be difficult for either of you. Once you've done that, just increase gradually until you've met your training goal. It's really mostly about going by feel. If you can keep track of distances and paces, that's great as well, but there's no cut and dry way to do it.
So...there's no "fitness test" that I know of for a horse other than getting out there, riding, and seeing how it goes. There are plenty of things you can check and do so that you can analyze their fitness yourself though :) Was that helpful at all?
A horse is the projection of peoples' views about themselves--strong, powerful, beautiful--and it has the capability of giving us escape from our mundane existance.