There are big differences in people’s sensitivity to bed bug bites. Some, about 10% of the Danish population have no reaction at all. This means that they can donate blood to a large population of bed bugs without noticing it, and maybe only discover the animals later by accident. In most people, the bed bug bites leave red itchy spots, an allergic reaction to the foreign proteins in the bed bug’s saliva.
Exactly because people react so differently to insect bites, it is difficult, if not impossible, to immediately distinguish one kind of itchy rash from the other, and the matter is further complicated by the fact that the bites can be mistaken for skin reactions due to hypersensitivity.
To get an idea you must take into consideration whether you are bitten indoors or outdoors, what time of day it happens and where on the body the clues appear. If you get bitten at night, on exposed parts of the body, maybe in a hotel room, there is a high probability that it is from bed bugs, but it could also be from mosquitoes. If a few fleas have gotten into the bed, they will mainly bite underneath the clothing, preferably where it fits tightly, or they bite where the body has been in contact with the foundation, since they would like to push back when they have to pierce the proboscis into the skin.