In the United States, liquor with 50% alcohol by volume or more would be considered 100 proof or higher. Proof depends on the amount of alcohol in the spirit as well as where you are. In the United States, proof equals double the percentage of alcohol by volume, or ABV.
Pay attention to the label as this is where the ABV will be listed as required by U.S. law. Usually, the label will also indicate the proof, but it is not a requirement. If no proof is listed, you can use basic math to deduce it. If a bottle of vodka is labeled 55% ABV, it is 110 proof.
Alcohol content can vary greatly, and there are many brands and types that boast an ABV lower than 50% (making it less than 100 proof). For example, the standard ABV for rum is 40%, or 80 proof, but that does not mean there aren't plenty of rums on the market with much higher proofs, regardless of which country's definition you go by.
Although the ABV measurement standard becomes more universally used, the "proof" remains a common term and has a long history originating in the 16th century. In order for British sailors to test the strength of their rum rations, they would pour the rum over gunpowder. If, after wetting it, the gunpowder would still ignite the rum had "proved" itself strong enough. However, if there was too much water in the rum and the gunpowder wouldn't light the liquor was considered "under proof" and weak.