There are many ways to tell if your hair is damaged. For one thing it is very obvious when looked at, because damaged hair tends to be very dry at the ends and lacks luster and sheen. There is no bounce and spring to the hair. It also breaks off very easily. It can also grow in very sparsely and be receding at the hairline. Hair that is damaged can appear frizzy and is very hard to comb through properly because it tends to get very tangled. Healthy hair on the other hand is thick and shiny and very resilient.
There are a number of hair products that are often overused and therefore can cause considerable breakage of the hair. Lye hair relaxers, for example, work by breaking the di-sulfide bonds in the hair. This is what straightens the hair but also causes severe damage. There are hair relaxers that don’t use lye, but they all damage the hair to some degree. Wearing tight braids also causes considerable strain on the hair root on an ongoing basis and will eventually destroy the hair root. This can cause significant receding of the hairline and is irreversible if not caught in time.
The best thing to do, is start building a better hair regimen and hairstyle for your mane. Try paying more attention to it and deep condition at least once a week just to get things rolling towards healthy hair.