From what stage can you speak of an inflammation?
From PubMed Health:


  When a wound swells up, turns red and hurts, it may be a sign of inflammation. Inflammation is – very generally speaking – the body’s immune system’s response to stimulus. This can be bacteria colonizing a wound or a splinter piercing your finger, for example. Inflammation happens when the immune system fights against something that may turn out to be harmful.


So when inflammation occurs, chemicals from the body's white blood cells are released into the blood or affected tissues to protect your body from foreign substances. This release of chemicals increases the blood flow to the area of injury or infection, and may result in redness and warmth. Some of the chemicals cause a leak of fluid into the tissues, resulting in swelling. 

Signs of an inflammation :

There are five signs that may indicate an acute inflammation:


Redness
Heat
Swelling
Pain
Loss of function


There is a loss of function, for example, when the inflamed limb can no longer be moved properly or when the sense of smell is worse during a cold.

This means that an inflammation does not start when a wound has been infected by bacteria, festers, or heals poorly, but already as the body is trying to fight against the harmful stimulus or a viral infection.

Not all five signs occur in every inflammation. Some inflammations occur silently and do not cause any symptoms.

So depending on the symptoms we can decide if there is an inflammation or no.