Although no one seem to have issued nay "official" guidance, merchant mariners plying pirate-infested waters, as well as naval personnel conducting anti-piracy patrols, seem to have developed some simple "rules of thumb" to improve their chances of spotting pirates.
@ Boat Size: Most pirates, whether in the northwestern Indian Ocean, off Somalia, or in the region around the Straits of Malacca, are using relatively small boats, essentially the same size as used by the local fishermen. So if a couple of fishing skiffs try to approach you, watch out. And be especially careful if they're moving at a good clip. That's because ordinary fishermen are loath to open up their engines, since fuel is money.
@ Crew Size: Most fishing boats have at most three crewmen. If there are more than that in a boat, or in nearby several boats, it's likely that someone's up to no good.
@ Fishing Gear: If you don't spot nets or other equipment associated with fishing, the boat may be looking for bigger game.
@ Birds: Fishing boats usually are accompanied by little clouds of sea birds; no birds, no fish, and that means pirates.
@ Fishing Grounds: Even in the relatively undeveloped regions where piracy flourishes, local governments usually maintain websites identifying fisheries, particularly those in their exclusive economic zones. If you spot fishing boats far from any of these, caution would be advised
@ Weapons: Even if there are only two or three guys in a boat, if they're waving weapons around, it's probably a good idea to avoid them.