Hey there! I'm happy to help!
I think that scientists can make connections between between modern organisms and their ancestors by noticing similarities in features between the modern organism and the fossils of the ancestor. If you look at a normal elephant and see that it has a trunk and tusks, you could see that this is similar with the fossils of a woolly mammoth, so scientists could infer that these mammoths were ancestors of the elephants.
Have a wonderful day! :D