Testing for blown head gasket

Opinions please from those who know far more than I:

What value does an emissions test have in determining whether a head gasket has gone? My garage missed the problem in my MG despite doing this test (and bizzarely still stand by this method, despite me having proven them wrong...).

I spoke to a second garage (an MG specialist) who said testing emissions to determine whether the head gasket had gone was worthless and they never do it.

Are they both right???