this requires the camera to have a HDMI port, and, a setting which allows the HDMI output to not show the camera settings
most modern cameras have this capability, for example:
Olympus OM-D EM5 mark III and later
Olympus OM-D EM1 mark II and later
Olympus OM-D EM1X and later
Panasonic GH-4 and later models
cameras that DON'T have clean HDMI out include (and it can be assumed in earlier dated cameras from these manufacturers probably don't have clean HDMI out):
for instance on Olympus OM-D cameras you go into the Menu to Movie section and find HDMI and change the HDMI mode from Monitor (which sends the camera settings) to RECORD (which sends a clean video)
on some cameras such as the Canon M3, you press the INFO button to turn off the displayed settings information
older cameras may be able to be used with work-arounds such as using AV out instead of HDMI but these will need a AV to USB adapter for your computer and need testing
it's is also useful if the camera's LCD screen can face towards you, the subject (ie. “selfie mode”)
not possible on current Sony full frame cameras
Olympus OM-D E-M1, E-M1x and latest E-M5 can all do this
it's also nice if the camera displays the video on the camera's screen as well as sending it to HDMI
