Most fibreglass rudders are two moulded halves round a metal stock, with webs welded to the stock inside and foam filling the rest. The stock enters through the top of the blade, and that joint never stays sealed for ever. Water gets in, sits for years and, in salt water, works on the welds that turn the blade.
How a surveyor checks it
Look at the stock joint and the seams. Practical Sailor's warning sign is rust-coloured water weeping from the joint: it means salt water inside and corrosion at the welds. Look for splits along the seam and cracks round the stock.
Run a moisture meter over the blade, accepting that rudders often read high. Then try to turn the blade against the stock, holding the blade while someone moves the tiller or wheel. Any movement means the stock or its welds have failed. Check play in the rudder bearings while you're there.
With the owner's agreement, drill a small hole low on the blade. Water that pours out confirms it; Practical Sailor fills such holes with epoxy and microballoons before launch.
What decides the grade
Whether the blade still turns with the stock. Movement is an A. Rust weeping or a split seam is a B, to drain and inspect the welds. High readings alone are a C.