This is what time their spawning pool will finish based on their build order (in-game time):
6 Pool ~1:49
7 Pool ~1:57
8 Pool ~2:04
9 Pool ~2:10
10 Pool ~2:18
11 Pool ~2:26
Additionally, the build time for a pair of
zerglings is 20 seconds, so their
zerglings will be on the field at:
6 Pool ~2:09
7 Pool ~2:17
8 Pool ~2:24
9 Pool ~2:30
10 Pool ~2:38
11 Pool ~2:46
Whenever you spot the incoming
zerglings (with your scouting
overlord), you can make an educated guess on what build order the opponent went for (based on what time it is in the game, and where you've spotted the
zerglings on the map).
All of that being said I think you should
drone scout. Unless you are Master or Grand Master, the potential scouting information you will receive outweighs the lost mining time of your scouting
drone. Plus if the other player did go for an early pool, you will have the better economy anyways.
It is good that you are positioning your second
overlord along the attack path, and you should be doing this with every
overlord in the early part of the game (until they have some sort of anti-air).
Build order & Defending
ZvZ, a pretty safe build is
zergling will spawn later than his, but you usually only have to stall for ~30 seconds. You don't necessarily have to throw down a
spine crawler. A
spine crawler takes 50 seconds to build, while
zerglings only take 20 seconds. When you're defending, don't be afraid to attack with your drones (and if your micro is good, pull back the weak ones), to stall a little until your
zerglings are out.
There are a lot of videos on youtube about defending against a 6 pools.
One last thing: while your defending against the
zerglings in your base, keep an eye on the minimap! Those
overlords you placed along the attack paths earlier are giving you vital information: if you see more
zerglings reinforcements coming, keep building more of your own. A lot of players lose when they fail to scout the reinforcements, and try to squeeze in some drones when they should been building more