Here is a 2009 Topps206 mini card of David Wright.
Here also is a 2009 Topps mini card of David Wright.
One of them is a base mini, the other is a short printed mini card inserted 1:20 packs. The SP variations in the main set are distinguished by a different no-numbered back. Let's take a look at these mini backs, shall we?
Hrm. Same back. Both have numbers. The only discernible difference in the two is the card stock is slightly whiter than the other. If you look at it edge on, you'll see it's white all the way through instead of gray like the normal minis. Hell of a way to distinguish the short printed mini variations, Topps...
The SP variation turns out to the be the smiley portrait version. Unless you were eagle-eyed enough to catch the card stock difference or had the variations memorized, you're kind of out of luck with these things. So if you have a mini and you're not sure if it's an SP, take all your mini cards and look at them like this:
You might find a diamond in the rough.