Before you open the hive look at the entrances. Much can be learnt about the internal state of a hive by observing them at the entrance and given bellow points gives you an idea of what you are looking for.
1) Robbing- Bees fighting at the entrance gate.
2) Poison- Find piles of dead bees at the entrance gate.
3) Virus Disease- Dead Bees still moving.
4) Period of the dearth or Lack of storage- Dead drones at the entrance or drones get removed by the workers.
5) Chalk brood disease- Mummified larvae littering entrance gate.
6) Possible Starvation- Dead larvae being thrown out but not carried away.
7) Healthy colony- Pollen being carried in to the hive by worker.
8) Orientation flight- Many bee's flying at the entrance. No fighting Bees facing the hive appear to be bobbing up and down.
9) Swarm- Many bees issuing from the hive in a swirling ascending mass.
Some of these observations may still a bit vague, but all will become clear. This inspection can be carried out every time you pass the hives, even if you don't have the time for a full, open inspection at that time. it takes only a couple of minutes. This may alert you if there is any possible problem in the bee hives.