First, check the resolution indicator on your TV's screen while playing a Netflix title. Many Samsung TVs have an on-screen display that shows the current video resolution, typically accessed by pressing an Info button on the TV remote. If the resolution displayed is 3840 x 2160, then the Netflix content is being played in 4K.
Alternatively, you can use the keyboard shortcut Ctrl+Alt+Shift+D (Windows) or Ctrl+Shift+Option+D (Mac) while watching Netflix on your computer. This will bring up a Netflix playback stats overlay that includes the current video resolution. If the resolution is listed as 4K, then the Netflix content you're watching is in 4K.