Fortunately clean climbing today is very different from back in the days, when people had to trust self forged pitons and obscure wood chocks.
Today there are plenty of well tested Clean Climbing Tools that offer decent protection.
What remains is the self-reliance for your own actions.
There is nobody who takes the responsibility from you by fully equipping a route.
One has to keep an eye on the placed gear, make sure the rope-drag does not remove nuts or slings and develop a feeling for which placements are trustworthy. For that, adequate training in form of courses is as necessary as sufficient climbing experience. For that you start your clean climbing career 2-3 grades below your personal performance limit and then step by step raise the bar. Clean Climbing at the limit is an art that requires year-long experience and real mastery.
Taking these basic rules seriously, clean climbing is as safe as climbing on bolts. Additionally one does not have to rely on the abilities and equipment of the developer who might have bolted the route decades ago. Was it done correctly back then? Are the bolts maybe rusty on the inside? In Clean Climbing these concerns are redundant.
Curriculum "Gear Placement"
There are several methods to learn how to use mobile protection gear. Trial and Error is one, but can hardly be recommended. Here is a little suggestion of how to get familiar with Cams & Co.:
Block 1 (1 h):
Getting an overview. What tools are there? What are the differences? How big of a force can they handle? What tool in which situation?
Block 2 (2-4 h):
Stay on the ground and place gear on the rock. Try placing as many tools as possible. Judge, test and improve your placements.
Block 3 (1-2 days):
Climb bolted single pitch routes (lead or toprope) placing as many pieces as possible. Test placements while descending. Fall into placements (backed up by bolts). Consider what could have done better at what point.
Block 4 (2-3 days):
Climb bolted routes using mobile protection gear and clip a bolt every now and then (in critical parts). Maybe climb 2-3 pitches in a multi-pitch route.
Block 5 (2-3 days):
Climb your first clean single pitch routes that are easy to protect (C-1 to C-1+).