I would put a brood frame with eggs and larvae from another hive into the one you suspect is queenless. I always put them in the top super in the number 4 position from the left (counting from behind the hives). That way I don't have to go digging in a hive and I don't have to try and remember where I put the diagnostic frame. You can check the frame in 4 days to see if they are drawing emergency queen cells. If not, there is probably a queen you missed. If they are drawing emergency cells, the brood smell will keep the hive from going laying worker and the additional bees will help keep the hive from dwindling. If this means that your worst problem is what to do with an extra queen in a couple of weeks, that is a good problem to have.