Depends...
If you get an idea of what your current mite drop is, you can estimate what the current level is. Stick the floor in for a week, count the mites. Go to
https://secure.fera.defra.gov.uk/beebase/public/BeeDiseases/varroaCalculator.cfm and stick in the information, that will give you an indication as to the current mite levels in the colony and whether you should be panicing yet.
If you don't have supers on you can treat with apiguard again, but as a 2-6week treatment it might not be practical at this time of year.
You can stick a Super frame in the brood box, the bees should draw drone comb on the bottom, when this is sealed, this can be cut out taking a good number of the mites with it. Run an uncapping fork over it to get the larvae out and see how many you're getting, these numbers can again be run through the calculator above.
You can also just uncap areas of drone brood in normal frames to get the larvae/varroa out and count that way.
And you can also dust with Icing sugar at the end of inspections. There appears to be some debate as to how effective this is with opions going from "not at all" to "quite" and everything in between. Everyone seems to have their favourite method of application but I just take a standard British handful and apply swift forward momentum in the direction of the top bars and then push anything on top into the colony. The theory behind this is that the icing sugar does two things, first it encourages the bees to groom and second, it coats the (sticky) feet of the varroa mite and makes it harder for it to hang on to the bees.