Investors often fall in love with the companies they own, which can be a mistake: If a situation changes, you need to change your opinion of the stock. But how do you actually know when the situation has changed? One way is to buy the products and services sold by the companies you've invested in.

Here's why it's a good idea, even though some might scorn the practice.

Buy what you know

Peter Lynch, who famously managed Fidelity Magellan Fund for years, wrote a book called One Up on Wall Street that suggests, basically, to buy what you know. So if you were to go to a mall and see that a store was constantly crowded with customers, it could be worth a look to see if it was public.

That same logic might be applied to Cava Group (CAVA 0.26%) and its fast growing Mediterranean restaurant chain, General Mills (GIS -0.33%) and its grocery products, Walt Disney (DIS 0.26%) and its amusement parks, or Netflix (NFLX -2.25%) and its content streaming service. You can see the products and services being offered, and experience what's going on at the customer end.

A person eating a burrito in a restaurant.

Image source: Getty Images.

The naysayers here would argue that you are a sample size of one, so maybe that's not enough to get a full picture of what's actually happening. And if you're buying from companies that you own in your portfolio, it could be a sign of an emotional attachment that might lead to bad investment choices down the line. Those aren't outlandish concerns, and you should take them into consideration. But there are ways around them.

For example, you could go to multiple Cava locations to see how they compare to each other. You could also go to nearby locations of Chipotle Mexican Grill (CMG -1.25%), after which Cava is styled, to see if Cava is keeping up with its competition.

As for falling in love with a stock, that's something you need to watch out for. It's important to ensure that you keep a division between the company's financial performance and its products or services.

A great way to follow your stocks

And yet, an excellent way to monitor a company is to keep tabs on its product and sevice offerings. For example, as a Disney lover, I've watched as the company's parks increasingly shift toward higher-end customers and away from families; while in the parks, I saw an increasing number of drunk adults after alcohol sales were increased in search of higher profits. I also suffered along with other customers as ticket prices were increased to levels I considered unjustified, given that long lines mean visitors might only get on a few rides in a day.

Those aren't good business trends, in my opinion. I once owned Disney stock; I don't anymore.

Recently I was in the grocery store and I came across General Mills' Häagen-Dazs brand in the yogurt aisle. I'd never seen that before, so I bought some of the flavors. They were (particularly vanilla) like eating ice cream that gets stored in the refrigerator and not the freezer.

When I was aware the Oui brand was coming out, years earlier, I made sure to try it as soon as I saw it in my local store. It was a good premium yogurt. I only know that, however, because I tried the product. My takeaway is that product development is a strong suit for the company, and I still happily own General Mills.

With Netflix, you might subscribe just so you can stream content. But if you own the stock, it would be shortsighted not to look at the service from an investment point of view every once in a while. With content an increasingly important aspect of the broader streaming business, as more companies look to directly reach consumers, it pays to monitor your own pleasure in Netflix's offering.

If you subscribe to another streaming service, compare and contrast. If you don't, find a free service and compare and contrast to that. Is there enough at Netflix to keep you subscribing? While I don't own Netflix stock, so far the company is still collecting my monthly subscription payment.

Lynch was right, in more ways than one

Using the tools at your disposal as much as you can is important when it comes to investing. And Peter Lynch's suggestion to buy what you know is good. But you can take it a step further: You can use your own experiences to find stocks you might want to buy, and as a way to keep up on stocks you already own.

If you feel a certain way, even though you alone won't impact the company's financial results, it is highly likely that others do, too. Don't underestimate the value you can derive from something so simple and seemingly trivial.