I do not know if he is reformed or not. I have his Classic Sermons Set which is sermon's by various pastor's. But then I'm not Reformed either. I was told that one can find some truth in diffrent authors.
As far as I can tell, Wiersbe is a "Reformed" Dispensationalist. My brother bought me the Transformation Study Bible by Warren Wiersbe for Christmas a couple of years ago. He is certainly a dispensationalist but I believe he is a monergist as well. Obviously I disagree with his dispensationalism but I find his down to earth preaching and writing style helpful.
You can find his sermons online. I listened to one a while back. He had some interesting illustrations.