As parasites are part of a highly heterogeneous group with a lot of different species each having different characteristics, they are able to cause a wide range of symptoms. Actually, some symptoms are very specific to some parasitic diseases while other symptoms are more common and can be related to many other types of illness.
However, it could be interesting to investigate the possibility of having a parasitic infection if you are experiencing weird and unexplained symptoms, especially if other medical tests do not succeed to find the cause of your symptoms. These symptoms could be varied and include unexplained and persistent gastrointestinal symptoms, fatigue, sudden weight loss despite increased appetite, sleep troubles including night time teeth grinding, skin rash, joint pain, unexplained mood swings and metabolic disorders. However, you have to keep in mind that these symptoms are not specific to parasites and could be linked to many other illnesses.
Moreover, it could be quite obvious that you have a parasitic disease if you find worms in your faeces. However, you should be cautious not to jump to the conclusion too quickly. In fact, some food is very hard to digest for human and can be found still intact in your faeces. Certain types of food can then be easily mistaken for worms. For example, it is the case of germinated bean sprouts that are a commonly used ingredient, especially in Asian cuisine. It could be easy to mistake germinated bean sprouts and worms in your faeces, especially with the rising anxiety and panic caused by this discovery. However, scientific literature warns us of this possibility and remembering what you ate the day before can highlight what you exactly found in your faeces.
In order to know if you got a parasite, it could also be useful to remember having a risky behaviour susceptible to transmit parasites in the past, such as travelling, eating raw meat, drinking untreated water, etc. For example, if you remember getting diarrhoea or being bitten by a bug while travelling in tropical countries, the likelihood that the symptoms you are experiencing are caused by a parasitic disease greatly increase.
Finally, if you think that you may have been infected with a parasite, the best and only way to know for sure if it is really the case is to have some diagnostic tests. However, as doctors are not usually thinking about parasitic diseases in the first place, it should be useful to share your suspicion with them. Actually, all behaviours susceptible to give you parasites should be mention when you consult a doctor in order to give them some clues about what to test. It could greatly reduce the number of tests and the time required to obtain a proper diagnosis.
Source: Centers for Disease Control and Prevention
Leung, F. H., & Watson, W. (2011). The parasite that wasn’t: A case of mistaken identity. Canadian Family Physician, 57(10), 1145-1147.