I recently bought a 5 gallon bourbon barrel that was dumped at the end of December, and when I bought it a week ago it still smelled very fresh and strong of whiskey. I know you normally have to treat a barrel, but I’d rather not do that here as I do not want to dilute the whiskey and oak characteristics. I have a big stout that has been fermenting for almost a week now, so knowing that the barrel was dumped at the end of December am I good to put my stout right in with out treating?
It may be perfectly fine. You’ll know if it leaks as soon as you put beer in it. 
One thing you can do is fill a tub with water and let it soak a couple days. You won’t know for sure if it leaks but you might swell the staves if they have dried out.