I should warn you that I own a large pack of imaginary dogs. They are all rescue dogs that needed a forever home, so it's cool. And I've taught them not to bark. Instead, they shark. Whenever they see a stranger they go, "SHARK! SHARK! SHARK!" It scares the hell out of people. But I can't take the pack to the beach anymore after they triggered that stampede, which killed more surfers than have all actual sharks combined.
So if you hear an imaginary dog yelling "Shark!" at the beach, and you don't actually see a shark, how can you be sure it's not an imaginary shark? Well, you might get lucky and find evidence washed up on the beach.
I guess I've probably found a couple hundred Dungeness crab trap marker floats on local beaches over the years. Exactly two of them had obvious shark bites. A float probably looks enough like food to be worth a shark's attention. Or maybe sharks are smart enough to simply be curious about odd objects in their environment. So if you are willing to eat multicolored marshmallows, don't get judgmental about sharks.
Also check any washed up soft plastic fishing lures. It's not uncommon for lures to end up with rows of little triangular divots that most likely were made by some small shark's teeth. Similarly, I found a chunk of Humboldt squid at Cape Mendocino a few years back that had scalloped wounds like it had been assaulted with giant pinking shears.
(Warning: The next paragraph is particularly nasty.)
Sea lion carcasses are worth checking for bites. I found a nice rotten one that looked like a marinated, maggoty mega Swiss cheese. It was pocked with holes that seemed shark-ish in origin. I found another adult sea lion bitten in half — nice and fresh with healthy-looking pink intestines tumbling out. I guess it could have been halved by orcas or a ship strike, but I'm going with white shark. And another time I saw a sad-looking sea lion clinging to the edge of an offshore rock with a big chunk missing from its bum. I waited but never saw a shark.
And you should definitely check washed-up whales for shark evidence. The sperm whale that washed up on Clam Beach in 2003, and the sei whale (I think that's what they decided it was) near Freshwater Lagoon in 2005 both had a bunch of bites on their bits. Some of the bites were big enough to have been made by adult white sharks, I believe.
Probably the most classic shark evidence is the bitten surfboard. Rather than washing up, these boards are always best when delivered to shore personally by the owner. This is what happened in 1990 when I happened to be standing near Trinidad Head watching a surfer, whom I'll call Mister S, paddling back out and then straddling his board. He was then roughly investigated by a white shark — launched into the air and flung sideways with his board in a rooster tail of seawater. I didn't actually see the shark as far as I could tell. But Mister S got back on his board and paddled in strong, thereby delivering his evidence-laden board to the beach in the preferred manner. And then later he was gracious enough to let me take a picture of the board in the parking lot of Mad River Community Hospital.
Mike Kelly writes other stuff as M. Sid Kelly on Amazon.