Observe the Moon at night. Its edge should be sharp and free from obvious false colour. Make sure you observe outside, and not through a window, closed or open. If the telescope fails either of these tests, do not buy it or return it for a full refund. The magnification of a telescope depends on the eyepiece used.
Telescopes usually come with a selection of eyepieces that offer low, medium and high powers.
Do not get carried away by advertisements for small telescopes that claim magnifications of many hundreds of times. Too high a magnification will show less rather than more, since an over-magnified image will be faint and indistinct. Of course, if the telescope's aperture is stopped down, the maximum usable magnification is correspondingly reduced. The atmosphere itself places a limit on the highest magnification you can use, because air currents make the images of stars and planets unsteady, an effect known as seeing . No matter how large a telescope you own, from a normal ground-level site the maximum usable magnification will be about 300.