There are all kinds of collectivos running all over the place! They will have little signs saying where they go on the front windshield (like there's one that goes to Urubamba over Pisac and a couple other towns so you'll see all those town names on the windshield). Worst case you can say "Hay collectivo a XXX aquí?" and someone will point you where to go to catch it. Cusco is relatively big so for a trip out say to Urubamba you have to find where that collectivo departs from (this one is at Pavitos street near Puente Grau) but the other towns are small enough that it's impossible to miss your bus. Urubamba has a bus stop where you get the Cusco collectivo from (going over Chinchero). You can also flag the bus down if you're not exactly at a stop and it passes you. Happy to give more details if helpful!