When I have issues that can't be resolved by documentary research, and also to seek further corroboration when I believe that I do have the documentary evidence, I create family trees based around the individuals concerned, and widen them as far as I can to include their siblings, their own and their siblings marriages, their offspring, etc. etc., bringing the trees as far forward to the present day as I can. You also need to include your proposed line of descent between the individual and yourself in the tree. I then upload those trees to Ancestry and other sites, and transfer my DNA test to them temporarily from my main tree, then sit back and wait a few days for the Thrulines for the new tree to populate, if it finds any matches at all.
Any matches between you and descendants of the target individual's descendants is a likely corroboration of their relationship to you. Of course, the absence of any matches does not confirm that there is no relationship, it may just be that no descendants have tested their DNA, but a positive result can help enormously.
I always run these as separate trees for investigative purposes so as not to clutter up my main tree with what might turn out to be irrelevant information. If I get a result, I can then transfer the relevant line that proves the link into my main tree. The tree has to be as wide and as inclusive as possible to maximise the inclusion of present day descendants, or at least their most recent ancestors, and hence the possibility of finding a match to one or more of them that links back via Thrulines.
Don't take Thrulines suggestions as gospel. They may or may not be correct and always need checking for accuracy and consistency, but if you've already got much of that information in a tree that you have researched yourself, you're more than halfway to corroborating any results that come up.
You can then transfer your DNA test back to your main tree at any time, but it has to be linked to the research tree for any DNA matching and Thrulines processing against that tree to occur on Ancestry's side, and your DNA test can only be linked to one tree at a time.
