Send your drone earlier - gas is usually built before the second supply depot, otherwise a tech block is hit. For banshees and other gas-heavy openings, the player can't afford waiting until 18+ supply to max gas - you should be able to scout what can be scouted before the second depot.
If you feel a need to keep watch on his base a while longer, you can always take the easy route - plant an extractor on his gas. At that point, he won't have the firepower to take it down quickly, it isn't too expensive for you, and if he's going for double gas, you've either slowed him down immensely or forced him to change his strategy to something gas-light. Really, it would be stupid of him to stick with a gas strategy if he was slowed down and hasn't yet made any irrevocable commitments. Banshees that come a minute later than usual aren't nearly as dangerous and require you to gimp yourself much less to defend against.
Consider keeping a ling near his ramp. Running up at any point would let you see all that you need to see. Siege tanks? Too many marauders in comparison to marines? No medivacs? Too few troops considering how far the game has progressed (aka he's probably cooking up a banshee surprise or something else he needs to keep hidden)? You'll see it all for the price of half a larva and 25 minerals.
As a zerg player, the third thing you do at the start of a match, after assigning miners and queueing up a drone, should be to send your overlord to the enemy base. This isn't 4:30-5:00, this is 0:03. This will let you know when the enemy barracks are being constructed - at that point, your scout should have left or be on the way, and the overlord should go to a place a ground unit can't reach, or even to your base. It will also allow you detect abnormally early gas.
