CAs check CAA records, clients don't.
There used to be a pinning scheme called HPKP, but the trouble with it is it's very easy to accidentally (or via malicious third-party action) end up in a situation where your web site becomes unreachable by people who've visited it previously because the key changed and the pin prevents their browser recognizing the new key as valid:
Thus, as a client, it is impossible to distinguish between:
1) A certificate that was issued in violation of a CAA record
2) A certificate that was issued correctly for a website that later changed their CAA record