By Paying attention to minute details...even microscopic. Learning from
someone that already has accomplished the learning curve. And by following
"feed all unknown varieties to your neighbor's dog"
To learn How to tell, get yourself the best field guide you can.
Seriously, there are not many species that look like this...
And there is only one mushroom that resembles this that could
be considered harmful and it's effects are mostly the Shiz....
Disclaimer: that is MY photo on mushroomobserver.org