A bit of a grey area defining a porcelain and ceramic tile Kevin, or am I missing some thing apart from a water test, ( not reliable ) scratch test not( really reliable) or having the good fortune of a spare tile at hand I would say its quite difficult.
I am no expert I know, but having spent lots of my time over the last few months gaining as much knowledge as possible regarding hard floors I have discovered this subject to cause many disagreements.
Now as we all know the normal route would be to install porcelain on floors as opposed to ceramic especially in high traffic area's so for me that is a good place to start hmmm, I guess with that last remark I may get ridiculed

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I am interested though in how you can tell a definitive difference?
Please .
Geoff