Aggresion, or play?

We have 2 male cats, one which rarely explores the house. Recently, he has been coming out though (yay!). Well, he is having some issues. I am watching the 2 males interact, and I can't tell if is aggression, or play. The male who doesn't leave his room (let's call him Red), slapped the other males back-end while he was walking by. I am unsure if claws were out, but I don't think they were. The other male (let's call him Blue) is a very playful cat. He often plays with our female. Red was an outdoor cat for a long time, but has not been outside in 4-5 years. Blue has never been outside. Red is about 10 years old, Blue is 3 or 4. Both of them are neutered. How can I tell what is play, and what is aggression? Also, if it is play, how can I encourage the behavior?

Based on what I've seen with my cats, I think it's a little of both. We recently added a male cat (Charlie) who is 2 or 3 to our household and the older two cats (Blue and Simon) weren't exactly thrilled at first. They are 6 and 7.  Once he was accustomed to Charlie being around the house, Simon started doing what your Red is doing.  Simon may be 7, but he's a Siamese and so very playful. When Charlie walked by, he would bat at him...and then hiss if Charlie took it as an invitation to play.  He did this a lot.  Poor Charlie was so confused for about a month. Now they play all the time, so you have hope  

There is no hissing going on. They know each other, and blue grew up knowing red. If it is play, which it looks like it is, how can I encourage it more? I really love seeing my babies playing. :3