I get this question all the time. As with many fly fishing questions, the answers can be simple and complex. This post is designed to be a mini-article rather than a treatise, so I’ll keep it simple.
If you’re asking “How do I know what bait is in the water?” you’re probably trying to match the hatch. That’s rarely a bad idea, especially if the stripers are keyed on a specific bait. Just be aware that while stripers can be selective feeders, they aren’t always selective feeders. So a generic baitfish pattern like my Soft-Hackled Flatwing may work just fine even when there’s nothing like it swimming around. Still, you want to know what’s for dinner, so…
Get a large fine mesh dip net and go fishing. Or, if you have a drag net and a partner, use that. You can do this on a beach, or off a well-lit dock and night, or in an outflow or an estuary. Sometimes you don’t need to get wet: get a bright light and shine it in the water at night. If the water goes nuts, it’s probably silversides. This first step is obvious, but many anglers are either too lazy or oblivious to do it. I’ll tell you in advance that you may surprised at the volume of small stuff that’s in the water.
Research the bait in your area. Get a field guide, or use the web for research. You should know what the resident baits are, and be able to ID them. A mummichog is not a silverside is not a peanut bunker.
Know what bait is likely to be when and where. Again: do your homework. Discover the patterns. Know when herring come up the rivers off Long Island Sound to spawn. Know when the sand eels show up in Newport, RI. Know when the cinder worms are swarming in the salt ponds of SoCo. Every year is different, but nature is always right on time.