Short answer: "なにか見えた?" is informal and "なにか見えたのですか?" is more formal.
In case I am speaking with my friend, I would say "なにか見えた?", on the other hand if I am speaking to my in-laws, I would say "なにか見えたのですか?".
"のですか" makes it more formal. more precisely "です" makes it more formal.
"の" or "か" doesn't have an effect to change the phrase formal. For example, you can say "なにか見えたのか?” without changing the meaning, and it is still informal, can be used for the conversation with close friends.
Putting "です" in the middle of it, you can make the phrase formal.