How to technically tell if a video has been sped up or not?
There is unfortunally (or maybe fortunally) no way to see if it is slowed down or sped up if done properly. What you have to look for is their movements. I'm not a guitar player, so I can't tell if it is real, but I think it sounded and looked real.

If you study the people there you can see that they move pretty natural. You can see on several occations that they blink in normal speed, but you also see that the main guitar player looks from place to place pretty rapidly. But from
 my experience with the piano you do everything a little bit faster when you play fast. Because you need to look at what you do, and when you are working hard with your brain and "slowing" everything down to keep up the pace you do things fast. The left hand finger movement also looked pretty natural. If it was sped down it would probably have been unnaturally fast and awquard movements.

Everything looked like it goes in normal speed, even their foots. Normally you have a constant speed on your foot when you tramp the beat. Depending on how slow you play you just increase the time you have your foot flat on the ground. Their foots didn't look like they went up and down too fast. If they had sped it up they would probably have had a small brake holding their foots flat on the ground before tramping the beat again really fast.

Secondly I don't see why a spanish TV programme would slow down their footage or why a good guitarist wouldn't be able to play that.