I have a 1959 ranch with a kitchen oven running off a 40 amp circuit breaker. I want to put in a newer slide in oven that says to have a 50 amp circuit breaker. How do I know if the wire is the right size (20 feet to the panel)? And do I really need to go to 50 amps?
It depends on what you have access to see. Typically wires are colored accordingly to their ampacity for easy identification.
For a 50A breaker, and 40A, you'd need #8 AWG wire so it should be sized correctly now. However this is assuming it's up to code, so it's best to check. Should be black coated wire. Again though, it all depends on what's ran. The wire should have the size printed somewhere along it.
If the device says to use a 50A breaker, then you need to use a 50A breaker. Otherwise it will probably trip constantly.