First you need to detect evidence of any enemy peon activity. If you see a shallows that leads to enemy territory, put down a radar house there ASAP. You'll need the house anyway and 30 wood is lot easier to obtain early than 100 wood for a dock which is usually inadequate to block the shallows w/out a 2nd dock. You must tell your allies anytime u see enemy peons in your area early cause they maybe headed for your allies and also they may be tooling more people than just you.
The following you must do 100% of the time when you see an enemy peon (ie just a flash of a different color on the little map to the right) 1) Build a rax near your woodies, possibly extending view into the fog, but far more importantly to allow a stable ASAP. Also you know have other option to fight back w/tool rax units.
2) You must have 150 wood for a stable as soon as tooled and spend 100 food for 1 scout. If you haven't found anything and still have alot of area to check make a second scout. An early scout is so useful, that this is almost my default strategy now.
I boat boom so much that I can often lose very little bronze time to build a scout. It is amazing how often that one scout finds and disrupts enemy building.
What you don't do is chase a peon you can't catch easily w/5 of your peons. That wastes tons of time. Chase it with one or maybe two at the most if you see it and can chase it at all... neilkaz..