You probably already found it, but if no, then check the top right side of the game screen.
You can see the amount of DEC you have in-game next to (on the left side of) your profile picture. Your profile picture is on the right side. If you move over your mouse to the amount of DEC you have, then you can also see your current capture rate, and if you click on the amount of DEC you have, then you can also check the amount of DEC you have on Steem Engine, and the amount of Tron you have as well. You can also transfer your in-game DEC to your Steem Engine wallet, or to your Tron wallet.
Image source: Screenshot (Steem Monsters).
Screenshot was taken on 2019.05.21, 06:12 CEST.
Image source: Screenshot (Steem Monsters).
Screenshot was taken on 2019.05.21, 06:15 CEST.