At the pinnacle of their popularity, Ty's collectible plush Beanie Babies were trading for hundreds or even thousands of dollars depending on their rarity. Like all fads, however, eventually the market cooled. Dedicated collectors are still out there, but market prices vary constantly. The best indicators of a Beanie Baby's value are its age and/or rarity and the condition of the plush together with its attached tags.
The biggest indicator of a Beanie Baby's worth is the condition of the plush itself, as well as the condition of its tush and swing tags. The swing tag is the tag hanging from the plush's ear, while the tush is the cloth tag attached to its bottom. The condition of the tags is as or even more important than the condition of the plush itself -- without tags in reasonably good condition, the plush itself is almost worthless. Dents, creases and other blemishes on the swing tag further reduce a Beanie Baby's value.
Age or Rarity
Since Beanie Babies were released in generational waves, certain plushes are much rarer and older than others. The rarer the Beanie Baby, the more valuable it may be. This isn't a hard and fast rule, however, so research each toy before investing. The age of a Beanie Baby -- assuming it isn't a counterfeit -- can be determined by examining the swing and tush tags. The styles on each have changed over the years, making the generation of a piece readily identifiable.
There are many price guides, printed and online, available to help with identifying the value of Beanie Babies. However, you should never take them at face value -- much of the information is speculative or otherwise obsolete and rarely accurate. The guides are, however, useful for determining what plushes are the oldest so you can identify them at a glance and research their current market value, or to build checklists to complete your collection.