Courtesy: Getty Images

Over the past couple of years, authorities in the United States have increased their scrutiny of people entering the country - whether they're visitors or returning American residents or citizens.

This scrutiny has taken various forms. Remember those famous "no-fly" lists? A number of other changes have been quietly coming into effect as well.

Canadians wishing to cross the longest undefended border in the world by air must now do so with a passport in hand. Previously, a driver's license or birth certificate was good enough. That remains the case for driving over the border (for the time being) but not for flying over it.

Likewise, Americans returning from Canada or elsewhere by air must do the same - show a valid passport when they return home.

Now, a new visa law comes into force, effective today, January 12.

It falls under what's known as ESTA, or Electronic System for Travel Authorization. In essence, anyone who comes from a list of pre-approved countries must fill out an online visa application at least 72 hours before flying to the U.S.

American authorities say the process takes very little time and approvals are usually done online in a matter of seconds.

They also say they have been approving more than 96% of all online applications.

While the process is not onerous, the trick is being aware of it in the first place. Travel agents are familiar with the new regulation - or they should be if they're any good at their jobs - but caveat emptor.

If you're planning a golf holiday to the U.S. in the next few months, be aware of the rule!