No, I dont believe you can tell just from an advert. Some dont even have a landline number. When I go out and quote, I take my portfolio that includes photographs and written customer recommendations/comments. A professional tiler should work to BS 5385. Ask one and see if you get a blank look or some er er er erms!

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Richard Knight Tiling
Quality Wall & Floor Tiling to British Standards
Member of The Guild of Master Craftsmen