For example, put a drop of special sealing wax or nail polish on the seams of your device. This way you can later determine whether it has been opened.
For larger devices, you can also use so-called "tamper tape". This is a special sticker that is destroyed when the device is opened.
"Tamper tape is also ideal for temporarily sealing USB ports. This way you can be sure that your device has not been connected to other devices for analysis. Ideal for border crossings where your device may be detained for a period of time.