The FDA Electronic Orange Book will tell you all you need to know.
Start here. Select how you want to search by the drug's brand name(proprietary name) or generic name (active ingredient).
This page for the brand/proprietary name searches.
After that you'll get a page full of the drug's applications. You may see multiple listings for each dosage size. You may see one listing for each dosage size. There might be just one entry.
If you see anything under the "TE Code" Column (usually A, B or AB) then the drug is already available as a generic. Sometimes it is available as a generic for just one application or even one dosage! If all dosages are available as generic for one or more applications you can, or will probably get the generic regardless of what sort of brain cooties you have. Otherwise click on a link with the lowest Application number (as that is the most likely to have the soonest patent expiration date). Still you should check each Application number (only one per dosage is required) just to be sure.
The next page has information interesting only for patent and approval purposes, but doesn't tell us much. Click on the link to view Patent and Exclusivity Info. That will take you to the page that tells us the soonest a generic version can be made.
The more important date is the patent expiration date. Nothing is going to happen before the patent expires. A drug may have multiple patents and that's the sort of thing that can hold up the manufacture of a generic.
Next is the exclusivity date. That's the FDA's version of a patent, more or less. The FDA awards a company exclusive rights to sell a drug for a various periods of time (depending on use and circumstances). Sometimes that extends the patent, sometimes it doesn't. Regardless, both have to have expired before a drug can go generic.
That's it. That's the absolute soonest a generic version of a drug can go on the market, after the original patent and the current exclusivity date (it can be renewed, but they get only so many), plus there can be lawsuits to slow down the process.
Patent use codes and exclusivity codes will tell you what the official application would be for the soonest generic if there are some wildly different dates, but it's not as if some insurance companies would pay attention to that.
Generics manufacturers will file lawsuits to get patents and exclusivity dates overturned. That doesn't work very often, but it does now and then.
Edited by Jerod Poore, 24 September 2009 - 04:15 PM.
Thought of a better way to check for generics & editted title.