When you're browsing inventory, check the product listing for a display's resolution: if it says FHD, full HD, or (1920x1080), the screen is in 1080p. If you see 4K, then it's a 4K-capable display. However, sometimes retailers will include the actual resolution (3820x2160) in the product title or specification overview. The first number is the horizontal pixel count, which doesn't matter as much, but the second number (2160) will always indicate 4K resolution.
If you want to know if the monitor you're currently using is 4K, there's a way to check. For Windows users, click the search bar and type in "display settings," then click "additional display settings" in the side bar. Under the "scale and layout" section, you'll see a drop-down menu of your display resolution which is defaulted to your monitor's native resolution. For Mac users, open System Preferences and click on Display; the defaulted selection is your screen's native resolution.