There's 4 Shrooms in total and they all spawn frequently on SHORE and seemingly less so on TROPICS. Being the Bugle, Button, Cluster, and Chubby. However of these 4 types, each one has a poisonous variant, with possibly an exception for the Chubby. Despite repeated re-runs of shore I've never encountered a poisonous Chubby Shroom. Do let me know if you encounter one. With that said, here's the 4 Shrooms with their Poisonous variants below.

(Top row is Safe, Bottom row is Poisonous)

You can see that the Bugle has webbing, the Button is spotted and doesn't have an X on top, and the Cluster has frills on the stalk. In most cases unless you're strapped for food, which is unlikely in Shore, it's best to leave the poisonous mushrooms alone. Consuming one will reduce Hunger, but will be followed shortly after by roughly 8 to 11 ticks of Poison depending on the mushroom. And eating multiple poison shrooms will stack this, meaning it's very easy to succumb to Poison.

(Before and After eating a Poisonous Button Mushroom)

If it's the achievement you're after, here's a few tips on getting it. Though in the end it's mostly RNG and will likely require multiple attempts. As previously mentioned, Shrooms spawn mostly on SHORE, the first section of the mountain. Mushrooms will spawn in set locations on the map, sharing a spawn with Medical Roots, however there won't be a guaranteed spawn at these locations each run.

(Same spot on two different runs)

This means you'll want to do a couple repeat runs of just SHORE, taking note of any mushroom spawns you notice, and ideally developing a route to quickly get to these spots. After that, it all comes down to luck on whether you'll get the 4 non-poisonous Shrooms to spawn in a single run. Happy mushroom hunting.

(Reward for achieving the Mycology Patch)