Plug it into a radio and listen is the quickest way. The unmarked filter on the top of the board will be active when the radio's mode knob is set to "CW".
Listen to the pitch of the noise in the receiver audio. If the pitch increases in CW mode, it's probably the AM filter. A wider filter lets higher audio frequencies squeeze through.
And if the pitch of the noise get lower, it really is a CW filter. They are normally too narrow to use for any voice mode.
But an unmarked part can be identified only by what it does. It will either widen the bandwidth of the receiver if it's the AM filter, or narrow it because it's a CW filter.