Read (High/Low) Arduino Output with an Raspberry GPIO Input Port
As you said, a resistive divider would be the easiest. Clamp diodes are cheap to add and provide extra protection.







simulate this circuit &ndash; Schematic created using CircuitLab

If you are using different power supplies for each of the devices and want isolation between them, you can use a common optocoupler such as 4N25, PC817, etc..







simulate this circuit