In the Lobby, you can determine the currency of a Contest by looking at the very right side of the tile for the listing of the contest.
OwnersBucks contest will display in blue font with an OwnersBucks icon, whereas real-money contests will display in green font with a dollar sign ($).
You can also filter contests between real-money and OwnersBucks.
Across the top of the Lobby, you will find the filter section. To filter by real-money (USD) or OwnersBucks you would use the options under the "Entry Type" header.
Tap "Lobby" in the bottom navigation bar and then tap "Filters" at the top. For the menu that slides in, you will have options under "Entry Type".