It is very difficult to understand if one has been affected or possessed by a ghost. Tell-tale signs do show up but unless one has a basic understanding/study of possession along with sixth sense, one would not even understand that it is possession.
We investigated the case of a person who abused his spouse at night becoming a very different person under the influence of alcohol. In the morning, his personality would be back to normal and he would apologise profusely or not remember the previous night. This is typical of a manifestation of a ghost, it’s just that people and their families are not trained to associate the behaviour with the likelihood of possession.
So, having basic understanding and knowledge of the subject of possession, what are its signs etc. help one to identify intellectually whether one is possessed. However, when the ghost has covered more than 50% of one’s consciousness, it is difficult for one to make out the presence of the ghost.