How to Find A magnetar
Whenever scientists look for magnetars, they mainly want to find unusual behavior when observed in X-ray wavelengths, including sudden outbursts of radiation, and occasional giant flares. When one sees a hint of magnetism in the atmosphere, they'll see a picture that looks likes this.
Then they'll know that they've spotted some magnetism but they still don't know if it's a magnetar. Magnetars usually have a certain amount of magnetism, and the X-rays scientists use can measure that.