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Sometimes it's possible to detect hot spots by comparing the temperature of two risers by putting your hand close to the metal. If one seems relatively hot, it may be an indication that the water passages on that side are clogged. If each side has an independent exhaust hose, you may also notice less water exiting the exhaust discharge. Note that the engine-temperature gauge may not indicate overheating when idling at low speed. However, all things being fairly equal on V-block engines, if one side becomes clogged, the other is usually not far behind. A more scientific approach is to hire an engine surveyor, who can use an infrared pyrometer to determine temperature variation in cast-iron exhaust parts. Plus, you profit from the surveyor's experience.
Another more visible sign of trouble is corrosion at the joint between the manifold and riser, which means the gasket between them has failed. If water is seeping outside the joint, there's a high probability it will soon leak along the inside joint to the gas-only chamber, if it hasn't already. Don't wait for confirmation on this. In fact, the best policy is to not wait for any warning sign at all, but to periodically remove and inspect the risers, and possibly the manifold, depending on what you find. A rough guide for inspection is annually, after three to five years in saltwater, and biannually in freshwater. If you see signs of corrosion on the outside, it's past time.
The good news is that a riser is usually not that difficult to remove for those inclined to do it themselves. Only a few bolts hold it to the manifold, and the exhaust hose is held on by a few hose clamps. Once off, you'll be able to see if all the water passages are open or clogged.
Take a large flat-head screwdriver and dig around for corroded metal that's easily removed. If you hit solid metal right away, it's probably OK, but if large chunks flake off, replace it. There should be no evidence of rust in the gas-only chamber on the side that connects to the manifold (expect to see black carbon). If there's rust, water is probably leaking in through a hole in the water jacket at the upper loop of the riser.
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Also look carefully at the metal faces where the manifold and riser join together. Because the walls of these faces are fairly thin to accommodate the water passages, the gaskets tend to deteriorate and develop leaks at the narrowest sections. Sometimes water seepage can be seen outside the joint, as noted above. If the metal face is pitted on either the manifold or riser, replace it or have the surface machined smooth again; otherwise, it will probably leak again, perhaps inside the gas-only chamber.
If you replace the risers (always replace both at the same time), you need to decide whether to also replace the manifolds, which are both more costly and a bit more difficult to remove. Replacing a riser or manifold typically costs several hundred dollars each, but failure to replace them when needed could cost thousands in engine damage. Any sign of rust in the gas-only chamber indicates a leak, which obviously warrants replacement, but it's more difficult to inspect the interior of a manifold.
Risers tend to fail before manifolds because the water is hotter when it reaches them. This accelerates corrosion and causes the salt to separate and stick to the walls more readily. Also, the top and aft sides of the riser are exposed to both saltwater and air where the two combine — a perfect environment for corrosion.
Unfortunately, there's no practical, conclusive method to evaluate the internal condition of manifolds. They can be removed, cleaned, and even pressure checked, but there's no guarantee they won't develop a leak soon after. In the end, you may have to weigh the cost of replacing an aging manifold against the cost of a replacing the engine if it fails.
Another option, possible on some engines, is to convert your raw-watercooled engine and manifold with a freshwater cooling-system conversion kit. This will increase overall engine life, and you won't have to worry about raw-water corrosion in the manifold. Risers, however, are always raw-water cooled, so they will still need to be inspected regularly.
A word of caution: Exhaust hoses disconnected from the riser provide an open path to the outside. They must be plugged and tied securely above the waterline to prevent water from flooding the boat.
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