My friend, who lived in Mexico for several years, tells me you can judge a Taqueria by it's Tacos Al Pastor - marinate pork tacos cooked on a rotisserie.  Only the authentic taquerias do it well.

We did those along with tamales, beef tacos, tacos de lengua (marinated beef tongue) and chili relleno.  The tacos were best, but everything is highly recommended.