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The best way to test this is by contacting your Local support and request a vacuum test.
Hi Liam729,
Is there any specific reason why you think it may not be? Have you damaged the device or simply curious?
Not at all jake I got it pretty much brand new off my brother when he upgraded to the Z5 he had it for about a year and kept it in fantastic condition just some of the stories I have read regarding the Z3 and onwards I just want to be very sure it's protected against accidents 
Hmmm... put phone in baloon and blow with with air, to increase pressure then check screen info? 
I see, then the suggestion above is correct, a service agent can perform a vacuum test to make sure it meets the required standard.
A quick suggestion, make sure to buy a Dock, to avoid wear and tear on the usb cover