this might be one way, if you look at your ancestor and who you have as the father, then do a wider tree for the father's family and see if you have any matches to any of those lines, then that should show if he's in the picture, I think.
Not having any matches, doesn't prove he isn't the father, as there just might be a lack of descendants tested, but if you found a match (or several), then surely it will confirm the right line.
Probably other ways to do it too.
hth
ps. with the situation you describe, might it be the case that you need to go back a generation or two further to find the link, the two possible candidates for father might be cousins or 2C.