Demai 4:2-3

Demai 4:2

If a person presses a friend to eat at his home and the guest doesn’t trust his host when it comes to tithes, he can eat with him on the first Shabbos based on the host’s assurance that it has been tithed. The next time he is invited, he may not eat at the friend’s house until the food is tithed. This is true even if refusing will make the host very angry at the guest.

Demai 4:3

Rabbi Eliezer says that one need not declare a portion of demai as maaser ani (the tithe for the needy) since an unlearned person is presumed to have taken this portion. The Sages say that one must declare a portion as maaser ani (since an unlearned person cannot be presumed to have taken it) but he need not separate it (to give to the poor because the burden of proof is on the one who wants to take it).