After having installed GETH using Homebrew on my Mac I manage to create a account, shortly after I copy my UTC file into the Keystore folder
I ran eth.accounts and it showed my new account still when I run
How do I successfully check my balance? is my account already linked now into GETH?
Yes, there should be nothing more you need to do. You can check your balance via the console:
web3.eth.getBalance('0x123abc'), but you'll need
geth to be up-to-date (sync'd with the blockchain) before that will give you an accurate result.
If you just want to check the balance, https://etherscan.io is an easier way.