Like you don't really know.  From moblues.org

1. If the guitar player has one small amplifier, Blues Band. 
   If he has a stack of them, Rock Band.
2. If the keyboard player only plays piano or organ, Blues Band. 
   If he's uses other sounds, Rock Band.
3. If the guitar player breaks a string and changes it on the spot, Blues Band. If a roadie comes out with a replacement guitar, Rock Band.
4. If the bass player's instrument has only four strings, or is a stand up, Blues Band. More than four, Rock Band.
5. If there are wives and children around the stage, Blues Band. 
   Scantly clad women, Rock Band.
6. If they play the same songs differently every night, Blues Band. 
   Play the same songs the same way every night, Rock band.
7. If they make up the lyrics as they go, Blues Band. 
   Read lyrics from a tele-prompter, Rock band.
8. If they arrive in a van, Blues Band. 
   If they arrive in a school bus, Rock band.
9. If everyone is drinking liquor, Blues Band. 
   Drinking beer, Rock Band.
10. If people dance together, Blues band. 
    Mill about separately, Rock Band.
11. If people clap their hands to the backbeat, Blues Band. 
    Pump their fists in the air, Rock Band.
12. If the male musicians are wearing Dockers, Blues Band. 
    If they are wearing Spandex, Rock Band.

Also......

1. Real Blues bands swing! No swing and all funk ain't the Blues.
2. You don't need electricity to play Blues.
3. You don't need Spandex pants to play the Blues either.