Most Banks have a sign out front of the building
Try exploring the insides of each of the buildings and look for a big metal box. The map Durango is a good small map to start off with and both banks have a safe right where you can see them (no hidden banks). Also, the defenders usually spawn near the banks. So if you play bots, start looking around after you win a match.
Also, try joining an online BR match. I think you will be able to find the gold by following the other players. Try connecting to Jeux, they got some good outlaws over ther (some good lawmen too so watch out!)