How do you know that in a Mendelian cross, the genotype TtYy is a dihybrid cross?

DD

Your question is ambiguous.  What you are showing is the genotype of a single individual.  Each gene is represented by two sites because the gene is on a chromosome pair.  Thus you show two genes, and each has a capitol and small case letter signifying that each is represented by a dominant and a recessive allele.  As it stands, this individual is a dihybrid....two genes, both heterozygous in genotype.

You ask about a "dihybrid cross".  Without knowing the other partner in this cross, one cannot answer your question.  If the other individual had all homozygous alleles, it would not be a dihybrid cross.  There are other combinations that also would preclude a dihybrid cross between your one individual and the other partner.