Were French people involved in the American Civil War?
The French General Lafayette allied with the American Revolutionaries. Lafayette's army scored several key victories for the rebels.
The answer is yes.

Is Capricorn the hypothetical zodiac sign of Satanism?
Satanism is a group of beliefs based on Satan and has numerous symbols. Satan (The Devil) is often depicted as the deity Baphomet, a Sabbatic Goat with a human body and goat head. The Capricorn is the zodiac sign symbolized by the goat. The Tarot card attributed to the Zodiac sign Capricorn is The Devil.
The answer is yes.

Is Central Park Zoo located on an island?
Central Park Zoo is located in Manhattan. Manhattan is an island.
The answer is yes.

Does frost mean that it will be a snowy day?

Frost forms regularly in areas that experience freezing temperatures and morning dew. Frost isn't deposited from the sky like snow, it forms on the ground.
The answer is no.