Raising to Gain Information

Raising simply to gain information is a tricky play and shouldn't be done often. Generally you should consider any information gained as an extra benefit of a raise you are making for other reasons.
There are occasions, though, when you cost yourself less by raising to gain information early than you would if you had not led your opponent into giving his hand away. These occasions usually occur in heads-up situations and only in early betting rounds. Furthermore, your opponent should be the type of player whose response to your raise is likely to reflect the hand he is holding. Otherwise your raise could very well give you wrong information.
What can you learn by raising? Well, if your opponent calls, he probably has a good hand. If he reraises, he probably has a very good hand. (It's for this reason you cannot raise to gain information when your opponent is the sort of player who is capable of a semi-bluff reraise.) If your opponent folds, that, of course, tells you he's weak, and you take down the money. An added benefit to raising to gain information is that sometimes your opponent may fold marginal hands that he shouldn't have folded.
You invest in an early raise to gain information in order to save yourself money later. If, for example, you call on fourth street in seven stud, you may continue to call three more bets only to discover in the showdown that you didn't have a chance from the beginning. But a raise on fourth street followed by a call or a reraise from your opponent allows you to play your hand knowing you're up against considerable strength. Depending upon your own strength, you can then decide whether and how long it's worth continuing in the hand.
Let's say with a pair of kings on fourth street in seven-card stud you raise an open pair of 9s. Your opponent reraises. You decide that opponent has three 9s and fold. By risking one bet (your raise), you save as many as three bets you might otherwise have called on fifth street, sixth street, and on the end. Your savings is even greater when the bet doubles after fourth street. A trial-balloon raise on a $10 round could save you three $20 calls later.
Nevertheless, raising just to gain information is tricky. For example, if that open pair of 9s just calls your raise, can you be sure that opponent doesn't have three 9s? What to do on the next round may still not be clear to you. That is why you should generally reserve your raises for other purposes and consider whatever information you gain from your opponents1 responses as an added benefit.