The Terumat HaDeshen (1, 294) says that which is the real Adar depends on the machloket (Nedarim 63a) between Rabbi Yehuda, who says that it is Adar I, and Rabbi Meir, who says that it is Adar II. Rambam paskens like Rabbi Meir but the Rif, Rosh and Ran pasken like Rabbi Yehuda, which is the general rule. You can access Rabbi Larry Rothwachs’ source sheet on the YUtorah website.
As for ways to be marbeh b’simcha, the Shaagat Aryeh (65) says that each person has his own simcha so you must answer for yourself. However, I would go easy on the meat and wine.