Any land is registered in a local township or county. You can go to the county court and ask who owns it. Also, in any land or house sale, part of the process is (or should be) a title search, and the title search will look to find any encumbrances on the property (liens, etc.) and any constraints on the deed. Don't let someone just "sell" you the property without a title search, and yes, you gotta pay for it.
You can find out if the land for sale is legal by checking out the land's title deeds. Make sure that the name on the title deed is that of the original seller and your real estate agent should also be there. Check out http://propertybytes.indiaproperty.com/?p=199 for details.