In my current fortress Goblin ambushes are more of a threat than goblin sieges. The ambushes often are on or across my short drawbridge before I detect them. How can I detect these attacks earlier (and preferably on the other side of my bridge)?
Cage traps are probably the easiest way, especially if you set up walls or other constructions that limit the path the gobs have to take to get to your bridge. Of course, traps are less effective if you have to cover a large area. (Also, if using traps, note that stonefall and weapon traps do work on ambushers, but do not display the ambush warning to alert you. Cage traps do.)
Another option is to chain up animals around the area you want to cover. War animals are preferred since they might actually do some damage, but any tame animal will trigger the warning when the ambushers come into its view.
A more ambitious alternative is to lock up your animals in towers. You can then access them by tunnel and need not worry about losing your animals.
Standard tactics involve placing spare animals at strategic points near (but not too near) your fortress entrance. Animals have a chance to detect, so that's better than nothing! An animal in a restraint will likely be attacked by goblins, so if you don't have that many to spare, you can build a square wall around the animal and carve fortifications into it. This will still subject it to archer attacks, but it's safer from melee ambushes. More info on guard animals.
Another trickier option is pressure plates: hidden ambushers will trigger them. The idea is to get creative in funneling any ambushers through one or a few narrow points where they have no option but to cross your pressure plates. This could trigger some kind of trap: water, magma, spikes, etc. Or you could simply drop a bunch of cats on their heads. A sneaking goblin suddenly finding himself with an enraged kitten latched to his face is sure to break their cover. (Actually, the cats will probably just detect them and then be spitted on spears, but hey you have plenty of cats.) Here's some inspiration.