It is hard to know for sure if you are having problems because of low potassium or because of some other cause. This is because the symptoms are not very obvious and can be caused by other things. For example, two of the major symptoms of low potassium are muscle weakness and muscle aches. These are not as easily noticed as some types of symptoms (like swelling or sores or things like that). They can also becaused by other factors.
The symptoms of low potassium are muscle weakness, aches, and possibly cramps. So you can only tell your body needs potassium by noticing these symptoms.
If the level of potassium is under 3.5 grams, our body starts to give some "signals", such as: fatigue, muscle's cramps, nausea, the loss of appetite, constipation, arrythmia.
But these symptoms, of course, not all of them, could describe another health problem, so, if you have any of these signs, it is better to have a complete set of medical test. It is better not to drag conclusions, especially when it concerns your health.
Hypokalemia (low level of potassium) could occur in metabolic disorders, flush vomitting or chronic diarrhea.