By using a gerber viewer... some people us the Gerbview in KiCad... there are several online ( http://svgerber.cousins.io/ and http://gerblook.org/ ) are just a couple. Your fab house may also show a gerber view before purchasing. Here in the U.S, I use OSH Park ( https://oshpark.com/ ) as both a gerber viewer and to purchase my boards. You get 3 board for $5 per sq/in, free ship world wide. Just check out the solder mask view. I always take a close look at all the gerber views before purchasing to make sure everything is correct... Sometimes your drill holes don't come out as expected... check those too.
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