send your military for scouting every season
How many times each season on average?
About 14-15 of the last month of each season. 3 times per year will do. Excluding winter ofc, as there are no caravans coming at winter. For my fortress, it's the only time when they can do some project job without ANY danger from outside ambushers. I care (too much i think) about my dwarves' safety.
But even this method doesn't always work. You will need about 10-12 WELL-TRAINED dwarfs to cover ALL your territory. I had 6 legendary fighters - but i had to increase my army size cause of one accident - my poor macedwarf got ambushed by 3 squads with 1 legendary mace-goblin and got crippled quite heavily (hell he not only survived and continued to serve as soldier, but took down about 4-5 goblins before they crippled him down with maces. Thx Armok i took my time to accept some really good doctors in my fortress - they had him fixed really neat - he had all his limbs broken and his right shoulder smashed).
Mark every entry points on your map and send there about 3-2 dwarves (never send them alone, even 1 mega dwarf warrior can get surrounded by 10-12 goblins and get struck down pretty quickly) and station them here for 4-5 days. Just before caravan comes (or even at this very time) goblins will send 1-6 ambush squads.
Just before you expect your caravans.....if you want to get the elves in then one as the winter ends to check theres nothing lurking.
Late summer, early autumn for the dwarvern caravan
Also, while a burrow for safety for your civilians is excellent at getting them inside and out of the way, assigning military squads to other specified burrows for tactical purposes is a little iffy, they often seem to ignore a burrow assignment given by their current alert. Adding the military manually, dwarf by dwarf through the c button when editing the burrow is more reliable but cant be easily toggled on/off
Ye, burrows are good. I NEVER let my civilians leave the perimeter wall, unless i am really sure there are no goblins outside (they don't have any business outside anyway), but you know, you can't be sure when those bastards might come