The United States minted circulating dollar coins containing silver for well over a century in a variety of styles, ending in the 1930s. Some U.S. silver dollars are extremely valuable as collectibles, but most fetch more modest sums. Finding the value of any particular silver dollar is a question of assessing its desirability as a collectible, which is mostly a function of its condition.
Understand the underlying principles of coin valuation. Simply being old doesn't necessarily add or subtract value to a coin, for instance, though this is widely believed. More important is how rare a coin is and even more important is the condition of the coin--how worn or circulated it is. Ultimately, like any other collectible, an old silver dollar is worth what someone is willing to pay for it and collectors are usually willing to pay a lot more for rare, new-looking coins.
Using the printed coin guides or websites, learn about the varieties of U.S. silver dollars, especially the ones most likely to cross your path. It's interesting to know, for example, that the "Draped Bust" dollar dated 1804 (but actually minted decades later) is one of the most famous coins ever made, so rare that each of the 15 extant examples has a name and pedigree. But it's more useful to know detailed information about the popular Morgan dollar series (1878-1921), which are relatively abundant and quite easy to buy and sell.
Make a rough estimate the value of any particular old silver dollars you might possess--or want to possess--by starting with "A Guide Book of United States Coins," nicknamed the Red Book in the coin collecting world. This definitive reference lists each silver dollar ever made by type, date, mint mark (the place where the coin was made) and grade (condition), offering a retail price estimate for each.
Study what factors earn one silver dollar a higher grade than another, since a coin's grade (condition of wear) makes great deal of difference when it comes to value, sometimes thousands of dollars. Best to worst, grades include Mint State (Unc.), Almost Uncirculated (AU), Extremely Fine (XF), Very Fine (VF), Fine (F), Very Good (VG), Good (G) and Almost Good (AG). Coins are also graded on a scale of 0 to 70, with 0 being a disk that used to be a coin, while 70 is perfect.
"Anyone with one good eye can get a fairly good idea as to the neighborhood of the grade of a coin," says Dave Hoer of American Coin and for most coins that's good enough to estimate a ballpark value. But for old silver dollars that might be very valuable, it's generally a good idea to get them evaluated by a number of coin dealers (who may make an offer for them) or have them graded by professional grading services such as NGC or PCGS, which charge a fee for their services.