You have two issues, first the obvious, that it likely will not hold water. Second is sanitation. A hard alcohol barrel that remains sealed will be sanitary because nothing can live in the high alcohol environment; but, if it loses its seal the alcohol will evaporate and microbes will sneak in. So, the first thing to do is inspect it closely to see if you have mold problems, if it has mold I wouldn't waste the time on that barrel, look for blue green penicillium (great for cheese, not beer) around the staves and bung hole. There is no way to know if you have other nasties like acetobacter, pedio, lacto, brett, or what have you. But, depending on what you want to do with the barrel those may be a good thing. If you are happy with the barrel and want to roll the dice on it you will first need to get it to seal. If you know for sure it won't hold water you will have to get the wood to expand, if it is minor it will leak for a few days and then seal up, if it is major you may have to expose it to a high humidity environment for a long period to get some initial swelling (which is too much work and not worth it to me). Once I know it is sealing I fill about half full with water in the 180 degree range and roll it around for a bit to surface sanitize, this will not kill anything that is deep into the wood though. At this point I consider it ready to be filled with beer. However, if you do decide to get it and find that it is very poorly sealed I have read that you can hit it with a high alkaline solution for a day followed by an acidic solution to neutralize it as a means of disinfecting. But, if you feel it would need such sanitizing before using I would pass up the barrel even if it was free.
