First you pass the FE exam, then you work for several years under a certified PE, then you take the PE exam. It's an 8-hour open book test, though IIRC for structural engineers it might actually be a 16-hour affair over 2 days.
If you pass, you get a stamp, which lets you approve drawings as a certified PE.
If you believe something is not designed to adequate standards, as a PE it is on you to not put your seal on the drawings. They can't build the bridge without an engineer doing this.
If you're taking about options when some other engineer has already approved the design, I'm not sure what the best course is. Maybe bring your concerns to the engineer in question and work from there.
Or if the design is fine but a contractor is cutting corners in construction and not building to the specifications, you could potentially engineer a new design to reinforce the problems, or you could make them take it out and build it right.