There are good ones, like the Habitat for Humanity mentioned above, but there are also the ones we see here on BF now and then (not so much recently) where people do want someone to fund their holiday.
One of the differences between the two is how much the person or people are willing to do. Are they willing to build houses, do promotional speeches and organise fund raising dinners, etc. etc. etc. along the way? Or are they just setting up a Go Fund Me under the name of a charity and then having a 3-month holiday?
Another is how much of the money is going to the charity. Is their website set up so that the money goes directly to the charity? Or does it go to them, out of which they snaffle a bunch for their holiday before sending a small handful to the charity?
Another is how well associated they are with the official charitable organisation. Does the American Diabetes Association or the American Cancer Society or whoever actually know about these cyclists? If the cyclists are cycling and working (doing the promotions, organising the dinners, etc. etc.) their way across the country, the charitable organisation should know all about them, and should be working with them.
So those are some of the things to look into.