The half-moon plug is right below the valve cover in the back of the head in line with the camshaft. You'll be able to see the black plug and the neighbouring bolt holes with a hand mirror and light. If you regularly check and clean your PCV system to make sure the flametrap, oil separator box and hoses aren't plugged then the odds of it ever blowing out are slim, but it's still a good idea to put a restraining strap behind it just to be safe. The usual signs that your PCV system needs attention are oil weeping from the front engine seals and the rear main seal. If you continue to ignore it then crankcase pressure can become excessive and blow out one of the seals or the half-moon plug, whichever is the weakest point. The rear main seal is not trivial to replace, requiring the trans to be dropped.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now