(author here) This is actually probably the hardest part of the whole thing - finding places to put the equipment and negotiating leases with all the right people.
If it's an actual cell phone tower you're looking at you can usually find it on the FCC website and find out who manages the tower. Most cell phone towers are managed by a third party management company (like Crown Castle or American Tower). The management company can usually tell you if there's fiber although their info is often outdated or just wrong. Even if the tower is owned and managed directly by a cell phone company they are still required to allow you to collocate (FCC!) but they will usually drag their feet through the process - plan on 9-12months and lots of red tape.
As for finding fiber - you can use a paid solution like fiberlocator.com. You can also drive around and look for fiber infrastructure, although admittedly it's hard to recognize if you don't know what you're looking for (or even if you do.) Frustratingly even when you find a company with fiber in the area they usually won't tell you where exactly the termination points are. You can sometimes give them a list of addresses and they'll give back yes/no for each one, though.