With metals do you mean stuff you need for synthesis or metals that you can sell to markets (like gold)?
If you metal like gold no you can't find it on planets (outside of crashed ships and other Pois that have cargo canisters lying around) only place to mine metals are planets rings.
If you mean stuff for synthesis its almost impossible to find something while in ship, because those rocks are so small. You need to be in srv and use the wave scanner to find what you want. There is some tutorials on how wave scanner works and what you can find and where.
This thread will help to learn what info wave scanner give you.
https://forums.frontier.co.uk/showthread.php?t=223453