In the scene after Rose Tico electrocutes Finn, where he’s layed out on the equipment cart, there is the subtlest showing of him being Force-sensitive. When after two attempts to move his stunned and immobile arm to show her the tracking beacon bracelet fail, he is successful on the third try. It wasn’t that he was suddenly able to move his arm. He moved it with the Force. If listened to very closely there is a very quick, rumbling bass sound effect before the arm moves. This sound effect has been used in Star Wars to indicate a manipulation or movement of the Force. It happens so quickly that it could be perceived as part of the sound of his arm moving. I noticed this when I saw the movie in theater the second time(I wasn’t looking for any clues, either).
Note: Please see earlier post “Finn is Force Sensitive” for further evidence in The Force Awakens.