All you have to do is look at the water......
If the seawater where you are collecting the bivalves is clear then they should be good. The Red Tide poison in only in the gut of those filter feeders and they suck in, digest and spit out that stuff fast. If you collect them yourself be careful with murky turbid waters like Cholla Bay as the Red Tide critters might be mixed in with the silt and mud stirred up by the tides. I've been mildly poisoned a number of times and it depends on how much of them you eat and the time it takes for it to affect you, which is usually within an hour or so. The effect of the poison, at least in my case, starts with a tingling in my neck muscles and shortness of breath and usually is gone within an hour or so. Now days I take my clams in small doses just to be sure. To sit down and eat thirty or forty of them all at once is asking for trouble. Until two years ago I would always buy twenty or thirty dozen of them off the street in El Golfo, but I just happened to get a load the was infected, didn't get me really sick but sick enough to say "nada mas", I'll just catch my own from now on.
As for the bigger bivalves like the Hachas (Pen Shells), Geoducks, Basket Cockles, Rock Scallops, etc., I only eat the meat of the muscles never any of the guts which is where the poison would be. Buying clams from the fish markets is playing Russian Roulette, I NEVER do it! If you do collect your own, here's a good tip, take two buckets. Use one to gather with and another half full of fresh seawater. Dump your catch into the water and you will see that with a few minutes they will open slightly and start pumping. Out comes most of the silt and mud then you will see their poop in the form of trails of greenish grey snotty strips. I'll swirl em around and refill the bucket several times and end up with some nicely cleaned clams.
JJ