I prefer a refractometer. I bought this one from Amazon, pretty cheap and works really well:
I wouldn’t leave the super off the hive without extracting. Depending on the honey, it can crystallize within a month or two, unless you freeze the frames (freezing delays crystallization almost indefinitely). That would really mess up the Flow frame mechanism! 
There is another option. Harvest the frames even if they are not fully capped, then test once you have the refractometer. If they are not nicely capped, I would harvest them in the kitchen over a large baking tray to catch drips. If the water content is less than 18%, you have ripe honey. If it is higher than that, you can always feed it back to the bees once the Apivar is done.
A further option would be to use Oxalic Acid Vapor rather than Apivar. You can take the super off for the 20 minutes or so that is needed for treatment, then replace it once the vapor has settled.
Another possibility is Mite Away Quick Strips. I imagine that you may be a bit warm for MAQS to be a good option, but I believe that you can use those with honey super in place. http://nodglobal.com/faq-maqs/