A short guide on dealing with players who use moles very early in the game.
Dealing with mole rush is 70% scouting, 30% not panicking. In order to deal with a mole rush, you have to first know it's coming.
As always, your priority during your first scout is to find out where your enemy is. Additionally, observe how many farms your opponent is building. Farms filled with brown earth are being built.
In order to build moles, your opponent needs food. Farms cost food, so an opponent who's built lots of farms won't have food to build moles. If your opponent builds farms, they probably won't mole rush. You're safe.
If you scout the opponent, and they do not have farms under construction, then the opponent is probably going to rush you with something--maybe moles.
BUT: be careful. An opponent can build farms to make you think they're trying to improve econ instead of rushing, then sell the farms and buy moles.
If you think your opponent is mole rushing, DON'T PANIC. You need to build something that will deal with the rush. If you have a bunch of farms queued, consider selling one to pay for your defenses.
You will need to build a T1 (Tier-1) unit for 60 food, or a Bullet Hive. Build it in such a way that it will be a target for assault instead of your farms. DON'T PANIC some more. You'll be fine. Fend off the attack, then either bulk up your econ or press the attack and nail their remaining farms.