"Traditional way" used to be a candle or a smoking wick, waved along the metal parts , to deposit a thin layer of soot. Some use lipstick, prussian blue or a black marker. A product called "Inletting black" or "Inletting gold" is available - depends on, if one has a light or dark wood grain stock. It does not dry out or soak into the wood. Leaves a good clear mark, yet cleans up easily. A small amount goes a long way. Use sparingly - apply with a brush. Be careful not to over apply or you'll get "false positives". A stockmaker handscrew set is very useful, applies pressure between the stock and metal, avoids damaged guard screws from repeated trial fittings.
