When you go via land you will talk to a CBP and should declare. By air, you fill out the blue form where you can declare anything. You are allowed 1 bottle of alcohol for age 21 yrs and over. See below for visitors to the US. When I have travelled by land with my wife we routinely take 3 bottle, 1 as a gift and 2 for the way back to avoid the long lines on the way back. Never had a problem and they know what you buy at duty free. You dont want to take a crazy amount. IMO CBP is more concerned about food (specially meats, dairy, plants and seeds) than a bottle of liquor.
Visitors and Tourists

For persons of 21 years of age or older: 200 cigarettes or 50 cigars or 2kg of tobacco or a combination of proportionate parts.

For persons of 21 years of age or older; 1 litre of alcoholic beverage
Including minors accompanied or unaccompanied: gifts not exceeding $100 in value, provided that the stay in the United States is not less than 72 hours and that the gift exemption has not been claimed in the previous 6 months.