How can you tell if someone's potassium is low (in the case of an eating disorder specifically) without a blood test?

See below: Symptoms of low potatssium level "hypokalemia" depend on the level and rapidity off fall. Muscle weakness, cramps and heart irregularities 'arryhthmia" are described. Since these symptoms are not specific only to hypokalemia and one can have low blood levels without symptoms, a blood test is necessary. Other electrolyte disorders can also coexist and are important to detect.

Hard to tell: Low potassium can cause symptoms if severe- < 2.5 or so. Nausea, ileus, weakness.