One can tell if one is in the tropics by observing the sun on both solstices. If, at either solstice, the sun hits zenith or crosses between the southern half of the sky and the northern half, then you are in the tropics. If the sun always stays in one half of the sky, then you are not in the tropics.

The tropics make an angle of about 23.5 degrees with the equator. This angle is the axial inclination of the planet; the angle between the equator and the ecliptic, or the tilt of the poles relative to the normal of the ecliptic.