North/West/South towards Aguascalientes/Guadalajara/Querétaro
There's a tollbooth right near the central de autobuses. Just take a local bus (5 or 6 pesos) toward Central de Autobuses and ask the driver to let you out before the caseta (tollbooth) - it's the last stop before the bus terminal, and you can see the tollbooth from the bus. It's about 200m away.
East towards San Miguel de Allende
Take a local bus towards "central de autobuses". Just before the bus terminal, get out at the huge roundabout (Glorieta de Sante Fe), the bus is going towards "central de autobuses", and you should take the other big road, just follow the signs saying "San Miguel de Allende". Walk ten minutes in that direction, and you'll arrive at a bridge for pedestrians to cross over above the road. There's a tope there and a little spot for cars to stop right after.
If you're brave enough, walk 20 more minutes in the same direction, the big road splits into two (the left one going to SM Allende, follow the signs), then walk another 10 minutes on that road, and you'll arrive at another tope, with a space for cars to stop right after and some small local restaurants around. Most of the cars here are going to SM Allende.
North towards Dolores Hidalgo
From the Mercado Hidalgo, take the road number 110 which is going North (the only big road with cars going North, next to the mercado), and walk 20 minutes on this road, following the signs which say "Dolores Hidalgo". You'll pass the Hotel Camino Real, continue climbing 5 minutes North, and you'll arrive at a roundabout with a metallic statue in the middle, and a Pemex gas station on the other side.
The Pemex is not ideal to ask for a ride, it's way better to thumb on the "Dolores Hidalgo" exit of the roundabout, there's a little spot for cars to stop too.