how to know if a mobile phone game (android) it's offline??

Oh god yes I'd love to know this too (although for the Amazon Store in my case). Too many times have I found a great-looking game, thought, "this'll be fun for my commute", and then discovered that it's online-only. :(

If it doesn't say in the description, the only way I've found is to look at the reviews and hope somebody's mentioned it.

upppppppppppppp anyone know that question???

try reading the description of the game or download it and play while not connected to internet

If it's free to play or has microtransactions, chances are it's online only.
You can usually tell from looking at the game's page.

Stupid answer , but doesnt it say when you install a game Requires internet connection to play or something of the sort ...
And if that tag doesnt exist ,i would assume the game can be played offline

They don't have a particular tag for that. Usually, games are pretty proud if they have an offline mode, so you should definitely check the description.
I'd say that a logical way to use the offline version is to actually turn off your wifi. I haven't seen any game with the particular option to play offline while you're connected to wifi.