Support of non-inline CSS styles (+ media queries) varies for client, device, and browser. The email client must support non-inline CSS in order for the media queries to work.
Here's a table of current devices and their support:
https://www.campaignmonitor.com/guides/mobile/#mobile-support
I would do this:
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Select 'Preview Email' for the email in question
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Then select 'Show Full Screen' under 'Preview Actions'
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Resize the browser window. Since nearly all modern browsers support media queries, this should tell you if they are in place.
To view on a device, make sure that you are using the device's native email client (the built in app for email), not the mobile website of gmail, for example.
Hope this helps. Responsive emails can be tricky!