I recently installed a medium sized preformed pond in a 12x12 box turtle enclosure in my yard. I was thinking about adding some red eared sliders once my fish population gets established.
I have read that box turtles should not be mixed with ornates or other non-local box turtles for health reasons. Would I need to be concerned with transmitting disease if I added some sliders?

yes, they can still spread diseases and parasites. Also how deep is the pond? Wouldnt want the box turtles fallin in and drowning.
Is there a way to determine that a turtle is healthy? As much as it would be cool to have a red eared slider, I'm afraid of spreading pathogens. You could quarantine, but for how long, and what would you look for? I would assume this would apply if I were to get another box turtle as well.
Also, box turtles can swim. They don't live in the water, but mine will get in and swim around. They float... the top of their shell doesn't even go completly under water. I have a ledge in the pond where they can crawl back out.
Quarantine for 6 months, look for common ailments, runny nose, swollen eyes, nose, ect.. But an animal can be a carrier and not show any symptoms so its really hard.
I would think that the main cause of concern would be transmission of parasites and disease. 3 months should be sufficient time for a disease to present (assuming that you're not dealing with an asymptomatic carrier, as blove points out). Two clear fecals at least
2 weeks apart should be a reasonable indication of being clear ofparasites.
Of course, none of this will 100% guarantee that you won't have anything transmitted from one animal to another, but I would consider it due diligence.