First, make sure the content you are playing is 4K. You can watch anything from The HDR Channel on YouTube. Once you find a video to play, hit Pause. With the video paused, click on More (represented by three vertical dots on the left) and then click on Stat for nerds. This will display the resolution of the content the TV is currently displaying.
Alternatively, you can check the TV's settings menu. For Sony Bravia 4K, go to Settings > Display & Sound > Video Output Signal > Resolution. This will display whether the signal is 1080p, 4K, etc. For LG 4K webOS TVs, go to Settings > All Settings > General > About This TV > HDMI Status. This will show the resolution of the TV. Additionally, you can check the user manual or the packaging box for the TV to see if it mentions UHD or 4K. If it does, then your TV is 4K. You can also check the screen resolution on your TV settings menu. If it's 3840 x 2160, then your TV is 4K.