There are a number of online resources available to troubleshooting GMail issues.
Google itself has a really useful site that you can use to test your setup - Google Apps Toolbox.
This will test and diagnose issues with respect to MX DNS records as well as validating SPF, DKIM, and DMARC records.
There is a also a trick to test that the spam filter is handling incoming spam as configured.
Include the line below in the message body in one line, without whitespace or line breaks.The Google SPAM filter will trigger on it:
This spam mail is included in a suite of tests provided by the website below. These mails include the EICAR test virus attachment and other tests which are used at the customer's own risk.