Although using a blacklight may not be the final say on whether an antique or collectible is authentic, it's a good place to start.
~ Determine if glassware has been reproduced.
Green Depression glass and Vaseline glass glow under a blacklight because the glass contains uranium oxide. I've seen both vaseline and green depression glass glow.
Other collectible glassware may fluoresce, too, but you'll need to do some research on whatever glassware you're collecting.
~ Detect repairs and paint touch-ups.
Repairs on porcelain may be so good you can't see it just by looking at the piece. But if it's been repaired, the glue will fluoresce. And if an item has been touched up or repainted, modern paint will also glow under blacklight.
~ Determine if old paper products are actually modern.
Old books, photos, postcards and other paper products made before 1940 rarely glow under a blacklight, but modern papers with bleaches and dyes will. Using a blacklight will help eliminate fakes and reproductions, but if you're about to buy a high-priced rare paper product, have it examined by an expert who will have additional ways to test its authenticity.
~ Date old fabric and determine if it's been repaired.
Modern fibers like rayon and polyester fluoresce under a black light. This you know if a quilt, old doll, vintage clothing, and other collectible textiles are modern. A blacklight will also bring out repairs by showing you modern thread or bits of new material incorporated into the old item.
Be aware that if an older textile has been cleaned with modern laundry detergents, this many make the item glow, too. Many modern detergents have additives that glow under a blacklight.
photo from flickr creative commons