A simple test is to take a sample from your heating system, either from the tank or drain tap, put the sample in a jar with a new nail (not galvanised), put the lid back on and leave it for a week or so. If the nail starts to rust you don’t have inhibitor in your heating system and need to add some.
If you have a small header tank in you loft (see picture) it is an unpressurised system and is easy to add inhibitor to the system.
You can just add it to the header tank and wait for expansion and contraction of the water to mix it into the system.
But the best way is to drain the system and refill with clean water and inhibitor.