While engaging in combat within Scythe, player can insert one combat card (yellow, small) per mech/character behind power dial, which serves as attack boost.
However, I fail to find in the rules book whether I am permitted to see values of my own combat cards and choose respectively. Both ways make sense (random choice and possibility to see), thus I am very uncertain.
You may always look at the power cards in your own hand and see their content and values. The content is only kept secret from other players. Other players only know how many combat cards you have in your hand.
From the rules page 7, up in the top right:
COMBAT CARDS: Draw the indicated number of combat cards. The number of combat cards you have is public information, but the content of those cards is kept secret from opponents.
Kept secret from opponents — not from you. You can see the content of those cards.
This means during combat you choose your power cards with full knowledge of what values they have. Your opponents won't know what you've played until the Reveal in combat (page 23).