It's rare that the electrical part dies. Most of the time the brass diaphragm dies, very rarely the aneroid dies, so if you suck on it & it holds air, it's probably OK.

I am assuming they have that hard yellow plastic coating over the screw for a reason?

It's epoxy. The big screw under the epoxy is the full throttle mixture stop & the small screw that some people adjust by drilling through the big screw controls the whole range. Internally, there is a part throttle stop making it adjustable three ways.

what are some of the symptoms of a bad one?

When the diaphragm starts leaking, fuel economy suffers. When the diaphragm fails completely, it goes full throttle rich, except in the idle position.

I liked D-Jet in my 164. Adjusted properly you can get economy as good as any other type of injection. However, if something fails like the MPS, throttle switch or distributor points, it's makes a great basis to convert to MegaSquirt.

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Three 164's, Two 144's, One 142 & a partridge in a pear tree.