I have a DH Gate special as well as the real one. When you compare them side by side, it's obvious which one is the fake, as the material and stitching are poorer quality, but the fake one looks absolutely fine when worn out and about.
Fake shirts are rife within the crowd at Hampden. 99.9% of those Italia 90 Umbro green and purple training shirts you see will be fake. The only 'official' ones are the Score Draw ones without the Umbro logo you can buy at JD.