Thunderstorms give many visual clues as to their strength.

In the upper levels of a storm (often observed at a moderate to far distance from the thunderstorm), you may see the top portion of a thunderstorm begin to spread out as if to circle the top of the thunderstorm. This is called the ANVIL.

Anvil clouds, which are mostly ice, form in the upper parts of thunderstorms. They get their anvil shape from the fact that the rising air in thunderstorms expands and spreads out as the air bumps up against the bottom of the stratosphere.

You can often see streaks of precipitation falling out of the edges of anvils. This light precipitation usually evaporates as it falls through the relatively dry air surrounding the upper part of the thunderstorm. The anvil is a tell-tale sign of a strong or even severe storm cell.

The photo here is a great examples of anvil clouds.