Hi.
I can understand why you might be worried about your mom if her boyfriend uses methamphetamine (crystal meth). Not only is the drug extremely addictive, but it is sometimes associated with violent or aggressive behavior. In addition, having either of these substances (crystal meth or marijuana) on the premises could put your household in jeopardy with legal authorities.
It may help to approach the situation this way: When you and your mother are alone, express your concern by saying that you found the drugs by accident. Show her the stash and where you found it. Tell her that you wanted to bring it to her attention for the reasons stated above.
Try to stay calm, keeping the focus on your concern for her and the possible impact the drug-taking behavior might have on the household. Verbally attacking her boyfriend may, as you fear, lead her to believe that this is a vendetta against him, making her more defensive than receptive to the information. Once she is armed with the info, it is up to your mother to decide about what to do. I hope that she will see the situation clearly enough to protect your home.
One more thing: You mentioned that you wanted tell your mother so that you could "see" whether she knows or not. Do you think your mother might be aware of his drug taking? Perhaps she knows, but doesn't know he stashes the drugs in the house. Or do you think your mother may also partake? It may feel awkward, but rather than wonder, it may be better to say directly that you are worried about whether she is taking drugs, too, and give her an opportunity to answer your questions.
Good Luck.
