When a flight attendant slaps a declaration form across your seat-back tray on your flight back to the U.S., you might know the general items you must declare -- but do you declare that new watch you bought in Switzerland? What about those Italian earrings? Knowing when to declare can be tough, but, as the U.S. Customs and Border Patrol says on its website, "If in doubt, declare it."

In the U.S., It's a Must

If you bought a watch or jewelry while you were abroad, you must declare it, but that doesn't mean you have to pay taxes on it. Most travelers qualify for CBP exemptions. If you traveled to anywhere other than Guam, American Samoa or the Virgin Islands, you will likely qualify for an $800 exemption. If you landed in one of those three U.S. insular possessions, you might qualify for a $1,600 exemption, but you still have to list any watches or jewelry you purchased while abroad.

Declare It Abroad

The rules for what you must declare at customs abroad will vary by country, but the general rule still works -- better to declare it to avoid confusion or hang-ups. Of course, you won't need to declare the watch or jewelry you brought from home, but, if you're bringing in several watches or necklaces as gifts or for a similar purpose, you're probably better off declaring what you have. Remember that you should declare any watches or jewelry you buy while abroad, even if you plan to wear them home.

Get Registered

You'll likely want to bring some expensive items, such as watches and jewelry, on your trip, and some of the most expensive of these can be foreign-made. If you're worried about getting through U.S. customs with foreign-made watches or jewelry you brought with you when you left the states, you can register them at a CBP office or the airport from which you're departing, reports IndependentTraveler.com. In fact, the CBP recommends this practice to avoid confusion. You'll need the items and their serial numbers to fill out the registration forms.

Considerations

Declaring items even when you're unsure can save you money. If you don't declare an item that requires it, the fine you could incur would likely be much higher than the small tax you would have had to pay if you exceeded your exemption level. Don't forget you should always declare watches and jewelry you obtained abroad, even if they're for your business, from duty-free shops or gifts you received. The duty-free status of items you purchase in duty-free shops only applies in the country in which you purchased them, so you might have to pay duty on these items in the U.S. if their value exceeds your personal exemption. As for gifts, you will have to estimate their value on your declaration forms, so aim for a reasonable estimation that won't draw attention for being too low.