I operate in an environment where, when possible, we conduct 50/50 operations (PF vs PM) - there is, however, no doubt as to who the captain is. Pre-briefing is a must - something along the lines of "The conditions are going to be a bit challenging - I'm sure you can handle it and I'd like to give you the chance to try, however if my comfort level is being stretched, I may take over - don't take this as a slight against your abilities, just understand that the responsibility is mine and I like to be conservative." This sort of "pre-brief" need probably only be given in such detail once with a new copilot - they understand the situation. Once the situation exceeds your comfort level, it can take more skill than one would desire to get it back on the rails. If you did end up having to take over, a post flight debrief would also be appropriate reinforcing the fact that you believe they could probably have handled it (if that's true!) but that your comfort levels were being stretched - if you didn't need to takeover, well, good job copilot!