Leading indicators that you are about to have some great tacos:
You just had a brisk 100-mile ride.
There are more Spanish than English words on the signs.
The picture on the sign is of a pile of raw meat.
Carniceria is one of the words.
Panaderia is one of the words.
You have to walk under an arch to get in.
Wrought-iron is featured (in this case a wrought-iron arch, so bonus points).
The wood chairs are all painted different colors.
An old man wearing farm-work clothes and a straw hat is by himself, reading the newspaper on the patio.
There is a dirty, beat-to-hell old Ford truck in the parking lot (this was true when I parked, but sadly replaced by a Hummer by the time I sat down with my tacos.)
There are more than three flavors of Jarritos in the fridge.
And proof:
Two el pastor, one carne asada. Two fresh corn tortillas per taco. With onion, cilantro, pico de gallo, guacamole, hot sauce and lime. The real deal.