There is no definite article in Czech, so how do you know that the correct answer is "this is the" vs "this is a"???
Actually, my understanding is that "to" typically means "that," where as "toto" brings it closer and makes it "this." Similarly, "ta" would typically be a feminine "that," whereas adding "-to" to make "tato" again brings it closer to the speaker and makes it "this." I'm not a native Czech speaker, still an intermediate student, but this is something I have agonized over. :)
In fact you can't, so both translations are possible. However for the given sentence the definbite article will be more probable in most situations.