It amazes me how singularly focused stripers can be on the bait they're after. My experience has been that not only are the bait species critical but the way they are feeding on them can sometimes be just as important.
In any case, this game is all about identifying the bait and what the fish are doing to eat it.
<b>Sand eels:</b> About the biggest hint is striper tailing on shallow flats. In deeper water, the flashing sides and downward focused feeding gives it away. When sand eels leave the sand and travel around in tight schools, they are hotly pursued and you can almost always expect a strike when they swim by in a hurried 'ball'. Sand eels are called candlefish and/or sand launces in other parts of the world. They can be almost as thick as a cigar and over 10" long on the outer beaches, and are a worthwhile forage for large bass at that size. Sometimes they will leave the sand and work in huge open water schools, feeding in a line hundreds of yards long / wide. This is something we see on Billingsgate Shoal each season, and a great time to be flyfishing. They can be identified by the multi-segmented wiggle and typical emerald color when swimming free, changing to a surprisingly dull sand color when buried. When walking on flats, they are often seen scurrying ahead of your footsteps for a few feet to re-bury themselves in a split second. Terns are a sure giveaway, when they dive for sand eels they come up with a sand eel mustache. When fish are attacking from below, they will NOT dive but instead pick the eels as they flee from the depths.
<b>Herring (juvenile):</b> When the bass start feeding on schools of young herring, pogies, or anchovies, they tend to get very focused on hitting ths schools as opposed to individuals... which can often result in a challenge for anglers. When the bass 'bust' a school of young herring, you will see silvery petal-shaped fish spray out of the water. As the school swims by, you can see the wide profile and silver sheen particularly when one or two turn on their sides to feed. The bait and thus the stripers move quickly, often making spot appearances along a shoreline. Herring and pogies tend to swim distinctly along the shore against the tide current, so it's often better to 'cut off' a school than to chase it from behind. Like all things in fishing, this observation is common but by no means a rule. All larger seabirds will get involved with a herring blitz. Gulls are especially tuned into herring, especially when gamefish are attacking from below. They have the slab-shape body, small ones the shape of a leaf (except thicker) with silver bright sides and a dark back, slight hint of purple along the union of these two colors and a dark spot on the gill (more than one on shad and menhaden).
<b>Adult Herring:</b> The biggest sign of herring (other than a stream with a run) is the presence of large herring eating birds like black backed gulls and gannets. Mid-late May is a great time to find big fish feeding on herring, and there are often huge bass ambushing adult herring in and around the river mouths where herring are running. They can be spotted in feeding activity by the small almost trout-like splashes they make, sometimes covering a hundred square yards or more. When adult herring get run by big bass or blues they make a wave of fleeing 10-12" bodies, often ending in swirls and splashes that do the size of big stripers justice.
<b>Mackerel:</b> Tinker and adult mackerel behave similarly to herring in that they move quickly, except mackerel are rarely seen involved in shoreline blitzes like the other forage species south of Great Bay (Piscataqua) and Maine's coast. This is not to say it doesn't happen, Sean Fields experienced an epic blitz of tinker mackerel forced into an ocean beach bowl and right onto the sand, pounded by huge bass and blues for hours on end... on Nauset Beach. Their brilliant blue-green backs and vermiculated markings are distinct. They have a very round profile (cigar-like) as opposed to the broad 'slab' profile of the herring family.
<b>Silversides:</b> Silversides tend to occupy an area and push their way around from spot to spot as opposed to the hurried flight of open area sand eels or herring. They have a uniform minnow look (tapered cigarette look), often a seafoam green color or a light tea color. When you hold one in your hand, they have a nearly transparent body with a distint, artificial looking silver horizontal band. The meat is so clear you can see it's anatomy. The eyes are medium small and they have a true minnow look about them. They are usually 3-4" but can exceed 6". They are found in thick schools and when the bass feed on these schools it's hard to get their attention with a single offering of any kind. Luckily, there are fish feeding beneath the schools on the wounded and others picking up strays on the perimeter of the pack. These are the easiest to catch in this case.
<b>anchovies:</b> I've seem chovies more commonly from Nantucket Sound south, although most times I wouldn't tell them apart from young herring with silverside colors, or silversides. In fact, if you see bait that look like silversides but have a slightly more herring like profile with a combined tea / green tint in the sound, they could very well be 'chovies. They have a distinctly clear body with the same silverside stripe - yet the bulldog face of a herring.
<b>crabs:</b> No need to describe these creatures, but I have seen two cases of crab feeding by stripers. One is the common upturned 'digging' and flashing of bodies as they pick up crabs. The other was a one-time thing where the crabs were herded in a wave by a large group of stripers against a long arching jetty. Many of the crabs were actually forced near the surface, and some were fleeing into the gaps between the rocks where we stood.
Anyway, chime in if you have observation on other forage species... I'll put some patterns in the archive to go with.