If someone had high or low potassium blood levels wouldn't an ekg show something abnormal?

Yes.: Potassium has a definite effect on ekg. Hypokalemia can result in atrial and ventricular arrhythmias while hyperkalemia can result in bradycardia and altered cardiac conduction, like peaked t waves, smaller r wave, a wide qrs, a prolonged p-r, a loss of the p wave, and finally a sine wave in the most severe cases. If untreated, cardiac arrest can occur, so the answer is definitely yes.

Sometimes: As potassium goes up, often the t-waves will go up in what is usually described as "peaked." a potential negative amplitude u-wave may show itself with low potassium.