You can usually tell whether a lens is positive or negative by looking through it. Because eyeglass lenses are usually weak, they don't enlarge or diminish very much. Positive eyeglass lenses can concentrate sunlight, like a burning glass. However, they are usually much too weak to set fire to anything. As the distance from the wall increases, the shadow of the frame seems to thicken and the bright area in the center gets smaller and brighter.
Negative lenses spread sunlight instead of concentrating it.
A negative lens combined with a positive lens removes some of its strength. In a real optical system, you usually get the best optical quality when most rays of light are roughly normal to the lens surface. In the case of an eyeglass lens, this means that the lens should be roughly shaped like a cup with the hollow side toward the eye. So most eyeglass lenses are meniscus in shape. Depending on the optical setup is, lenses can act as magnifiers, lenses can introduce blur, and lenses can correct blur.