You can get busted for illegal substances, legal substances that get you high (alcohol, ayahuasca, salvia), things that look like drugs, prescription drugs that aren't yours, more than a week's supply of your own prescriptions, and any paraphernalia, such as shotglasses, scales, or empty bottles. Tobacco and caffeine are okay.
You can get busted for having substances, supplying them to someone else, being messed up, or smelling of them. RDs can search your room or car without asking you. They may open unattended bags for the purpose of determinging whose they are; in theory, a name on the outside should make opening the bag unnecessary. They cannot search your person.
You can get in trouble for being in the same room as drugs or for drugs being found anywhere in your room or the common space of your suite, even if they aren't yours or you did not know, though claiming such may help your defense. It is up to the school to decide whether you ought to have known and whether or not you actually did. It's good to talk to your roommate about what is allowed in your room. If you find yourself in a "sketchy" situation with loud people engaging in forbidden activities, especially while breaking parietals, you may want to leave before the hammer falls. If people are doing any of these things in your room and you begin to worry about getting busted ask them to leave (in a kind manner, no need to yell or get angry) or say "Hey, guys, how about we go [insert place here]". As a general rule, RDs will bust everyone who is around when they bust anyone.
RAs and RDs know that students do things they are not supposed to, so please be respectful by not waving it in their faces. RDs may decide to knock on your door and say hi, so don't keep any contraband out in the open, and certainly not near a window. Anyone smoking anything indoors, smoking pot in the open, or being visibly intoxicated or smelling like alcohol in public is asking to be busted. Don’t talk loudly in public places about sketchy plans, and especially not about who is selling. Hypothetical conversations and stories about how drunk you got on your sister’s birthday are fine. RAs cannot punish you, but they can tell an RD about your behavior.
If you must smoke pot, stay out of sight in the woods during the day and well away from the dorms at night. Clinking bottles (in your backpack or room) are another dead giveaway, so plastic might be the way to go. So is puking, especially in a public place (though the ditch outside Kellogg is luckily not too public), even more so if your puke will smell like alcohol. And for goodness sake, wipe under your nose.
If the administration feels that one sort of bad behavior or another is becoming especially endemic, they may start cracking down. For example, when cocaine use briefly exploded during the 2003-4 school year, administration responded by expelling the people who brought it to campus, turned other students onto it, and distributed it. Please don't bring hard drugs to Simon's Rock.
One sure-fire way to avoid being busted if you're not inside is simply to run. For example, say that you're in the woods doing something you shouldn't be and you see an RD coming towards you, just run away. Then later if anyone tries to talk to you about it say it wasn't you and make them provide proof that it was you that they saw (and not that guy to whom you lent your jacket) and that you were doing anything you weren't supposed to be. If you're inside and an RD knocks on the door shove everything in a backpack and jump out the window (may not work in some parts of Hill House). One student followed these two rules for four years and was never busted for anything despite being actively hostile towards many of the RDs and flaunting disrespect for the rules in public speeches and community council meetings.
If running is not an option, or like some unfortunate students, you run the wrong way and very closely run into a security guard, shove it up your sleeve, or in your shoe if possible, if not possible, throw it when their attention is diverted. Deny as much as possible, if caught with someone else, it is better if one person admits to more. Tell then you were with townies, not other Rockers. Giving away names when caught will ensure a deep hatred from many people.