Consider this answer to an interesting question. It's not possible to prove completely that someone is the descendant of someone else, there's always a nonzero probability that the results are a coincidence.
How confident can you get that genome T is the descendant of four unrelated grandparents? How would you tell?
Restrictions: You're not allowed to sequence all other humans and rule out other possible parents. Time traveller is has the normal number of chromosomes. You can test people's parents if that would help.
The maternal grandmother would share mitochondrial DNA, and the paternal grandfather would share the Y-chromosome, or the paternal grandparents would have one X chromosome between them. I'm not an expert on SNP accumulation but would there be enough information to conclude this person was from the future? Would there be too much mutation in two generations to pick out the relationships under it?