How do I know it is a bee and not a wasp or fly?
Flies are easy. Flies have one wing on each side. Bees have two. Flies also tend to perch with their wings pointing out an angle. Bees tuck them away.
Telling wasps and bees apart is much harder. Bees tend to have wider bodies and appear more robust. Bees are usually hairier and you can often see where they are carrying pollen. However, it turns out that bees closest relatives are a group of wasps and these wasps are very difficult to differentiate from bees. The easy way to tell bees and wasps apart is to use a microscope and look for a branched hair. Bees have branched hairs, wasps have simple hairs. Alternatively, watch what they eat. Bees are vegetarian, wasps are carnivores.