The quickest way to check: Copy/paste the address into a block explorer like https://blockchain.info. Just paste the address into the search field and submit. If the address is valid and has transaction history, you'll be shown the transaction history. If the address is valid and has never been used before, it will give you a message like "Address has never been used on the network."

If the address is invalid, it will return one of these: "Checksum does not validate" or "Oops! We couldn’t find what you are looking for.
Unrecognized search pattern. Please try searching for a transaction by entering a block height, address, block hash, transaction hash, hash 160 or ipv4 address."