1. Antiques are, by definition, at least 100 years old. Anything else is “vintage” or a “collectible”.
2. Move the piece away from walls and other items so you can get a good look at all sides, including the bottom. Study the piece under bright light or ask to take it outside in the daylight.
3. Examine the finish for natural signs of wear. Authentic pieces will have dirt in crevices or in places that are hard to clean, and wear around knobs and on drawers caused by friction. The absence of such wear may indicate that the piece is not old, or that it has been refinished. The latter is not necessarily bad if it’s been done well, but it will lower the piece’s value in most instances.