Knowing the gas tank has to come off to access the exhaust donuts, that will be your first challenge of being able to start the sled while you have everything off. As for determining if they are bad other than a visual inspection, if they are real bad meaning there is hardly any donut material remaining, a telltale sign would be that your flex pipe/donut clamps are loose and can be spun.
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The reason guys like copper donuts is that they will last much longer (ie. not deteriorate like the OEM ones) and just like the OEM ones, they too go on dry as you just slide them onto the flex joint.