A smelly head is the complaint. On survey, the questions are whether sewage is getting out, where, and whether the tank can still take a pumpout without splitting. Practical Sailor's finding is that the smell usually comes from hoses and fittings rather than the tank itself.
How a surveyor checks it
The tank. Look at the tank, its seams and its fittings for weeping, and the bilge below for staining. Note the material. Peggie Hall, quoted in Power & Motoryacht, says urine is so corrosive that even the best metal tank typically starts leaking at a seam or fitting within two to five years, and after about ten the bottom looks like a colander. Her recommendation is rotomoulded polyethylene with at least a 3/8 inch wall.
The hose ends. Practical Sailor lists slow leaks at fittings and clamps, and sewage that was never cleaned up, as frequent causes of odour. A crooked clamp or scored barb leaks however tight the clamp. Sealant smeared on a joint is a temporary fix.
The damp rag test. Practical Sailor: scrub the hose clean, wait several days, wipe it with a damp cloth and sniff. If it's permeated, the smell comes back. Good Old Boat's author used the same test and found the hoses weren't the problem.
The vent. In his case the culprit was the 3/4 inch vent hose, reinforced plastic tubing never meant to hold sewage odour, and a through-hull fitting completely blocked. A blocked vent lets pressure build when the head is flushed and can force a leak somewhere in the hoses. At a pumpout, with no air coming in, the vacuum can implode the tank. One clue he missed: the head's flush handle rising by itself.
Where the vent comes out. Practical Sailor notes a vent in the bow near a hatch blows the smell back into the boat on a breezy day.
What decides the grade
Sewage escaping into the boat, or a tank pressurised by a blocked vent, is an A. Permeated hoses, a metal tank and weeping fittings are B. Wrong vent hose or a badly placed vent with nothing yet escaping is a C.