I’ve always believed you can tell a lot about a person by how long it takes him to reach for his wallet when the check comes. Or whether she watches Netflix or PBS. Or what color he dyes his hair. Or what books she has on her bookshelf.

People I considered intellectuals seemed as bright as Homer Simpson after I saw their libraries, even though they were prominent doctors, lawyers and politicos.

I expected to see Plato, Herodotus and maybe Thomas Aquinas, but instead I found J.K. Rowling, Danielle Steel and Harold Robbins. That’s why I don’t let anyone peak at my books because they’ll find X-Men comics. To compensate, I keep a copy of Sartre’s “Being and Nothingness” in the bathroom ... for show.

Another way you can learn about people is by snooping around their medicine cabinets, but you have to combine the stealth of James Bond with the curiosity of Sherlock Holmes because you don’t want to get caught rummaging through pill bottles when you’re supposed to be on the toilet, reading Sartre.

A medicine cabinet tells you more about someone than their voting record, their car, their Facebook page and their tax returns. I’m sure President Trump would agree.

When we have guests over, and they’re in the bathroom too long, I tell my wife, “Go knock on the door and see what they’re doing in there. They’re probably casing out the medicine cabinet.” And if I hear a pill bottle hit the floor, I rush up the stairs, ready to break down the door.

I’ve been paranoid about people rummaging through our medicine cabinet ever since someone made off with my teeth whitener. As a result, I’ve been checking everyone’s teeth, without exception, until I find the culprit. I just hope it wasn’t the pastor.

Now, when guests come over, I tape the medicine cabinet shut — and not because there’s anything illegal or questionable inside. There are no drugs or little blue pills or anything like that. I just don’t want them to see what IS there.

I took an inventory recently and was shocked to discover what my family has stored on the shelves. There’s pimple medication left over from when my daughters were teenagers, but I save it because you never know when you’re going to need it again. I still get pimples, even though I was told a long time ago that acne would end when adolescence did. They lied.

Can you keep a secret? I also found rose hip oil, manuka honey cream and something called chia seed oil. I’m flabbergasted. Who bought this stuff? What do they do with it? They must have learned about these New Age concoctions from watching Oprah, Dr. Oz or the Wizard of Oz.

This is why we NEVER have the boss over for dinner ... because he’ll want to use the bathroom and might do some snooping. My greatest fear is that anyone who looks in our medicine cabinet will think we’re growing cannabis under fluorescent lights in the basement. We’re not, so don’t call the Feds.

It’s nothing like my parents’ medicine cabinet. They kept prescriptions in there for decades and had some of what may have been the original batch of penicillin. They each took about five pills a day, which they kept in plastic organizers so they wouldn’t forget anything. I inherited that problem and can’t even remember to take my One A Day multivitamins or the stuff I drink to lower my cholesterol. I also eat a couple of bowls of Cheerios to keep the LDL down, but the Cheerios box won’t fit in the medicine cabinet.

My biggest fear is that our privacy as Americans is being threatened, and the FBI will start investigating our medicine cabinets. Our legislators are drafting laws to protect us from Facebook, Google and Amazon, along with Siri and Alexa, but who will protect our medicine cabinets from an invasion of privacy? Think about that the next time you reach for the chia seed oil. No one is safe.

Joe Pisani can be reached at joefpisani@yahoo.com.