If you aren’t sure what development cards your opponents have, assume that it’s a victory point card (because even if it isn’t, it will likely help them acquire at least 1 victory point when they play it).
In addition to your own development cards, it’s also important to keep track of your opponent’s development cards, so that you know how close everybody at the table is to winning. If you aren’t sure what development cards your opponents have, assume that it’s a victory point card (because even if it isn’t, it will likely help them acquire at least 1 victory point when they play it).