Condition
Examine the condition of the records. Records with scratches that will cause the needle to skip when played are automatically worth much less than those that are in perfect condition. The record should be housed in its original sleeves. Typically, records were sold in paper sleeves which were inserted into color printed jackets. Both covers should be present, legible and free of major damage. Even if the record itself is in good condition, the value can go down if the original sleeve is lost or damaged.
In order to determine precisely what condition your old record is in, you must "grade" it. There are several possible grades: Mint (M), Near Mint (M-), Very Good Plus (VG+), Very Good (VG), Good (G) and Good Plus (G+) and Poor (P) and Fair (F). Mint represents perfect condition, and the record loses value for each subsequent grade.
Other Factors
Scarcity of a particular album is a major factor in record prices. This depends on how many copies the record company initially produced as well as how many remain on the market today. Demand also plays a large role in how much a record is worth. If a particular artist still has a large fan base, the record will likely be worth more to potential collectors.
Your record value could be altered if it is an original release or a promo. Original releases are typically the most valuable and often include special inserts that later editions do not. Promo albums were handed out for free to radio stations or for other promotional purposes and typically aren't as valuable as original releases. Historic events can make prices of records by certain artists skyrocket, for example, when an artist dies.
While some people believe that the older the record, the more it will be worth, that is rarely the case. Availability or lack thereof often determines desirability. The vast majority of records were produced in the millions and aren't worth much now.
Price Guides and Other Resources
After you have evaluated the condition, rarity and demand for your record, consult a record pricing guide. These guides provide market value estimates for albums and should give you an idea of how much money your record is worth. Several sites such as Popsike.com and Musicpriceguide.com are free, while other sites like Recordmaster.com require a subscription. You may also purchase printed guide books, though they may not be as up to date. An expert at your local record store can also be a good resource.