Answer this as sarcastically as possible

Put a bottle of water near the dead guy, and leave him there for a few hours. If he doesn't drink it, he's either dead, or completely retarded.

When he doesn't make another Youtube video or leave a Twitter post in one week. That is a certain certification of death
( I changed my answer because this sounded more funny :P)

When they get up and start walking around like a zombie trying to eat our brains! Would that make it too obvious, or do you need more evidence?

When you're able to scroll through their web history in front of them without a single reaction from the dead person. xD

you sit on their gut for a few hours and if they don't move nor make noise then you truly know they are dead...

............

When you tell them to talk to you and don't answer, or move, for a few days, then they MIGHT be.... dead! ☠

He/She is not alive.

Oh, i don't knooooow, definetely not by, y'know, by the fact by checking their heartbeat and breathing and not hearing anything, not by that. They need to rise from death like zombies and they need to have zombie babies, and those zombie babies need to dance to Michael Jackson's Thriller even as fetuses. Y'know, normal stuff...