[crAzerk]

Don't understand your question, didn't we already answer 'how do I know he went gas first' ? Just check the gas left in his Refinery!

A little trick some Terrans do is they actually build their factory further away from ramp (so your ling can't see it even when you poke up), and they wall off with 3 supply depots instead. In that case, if you don't get the early scout in you just have to coinflip and hope for the best. That's why I prefer to scout at 10 to be able to get a feel of what's coming.