Let’s tell you one way to check the trace quality is good or not after developing of flexible circuits. After developing, even if some developing trace is dirty, or the base is long, is unable to check them out with a magnifying glass; but Cucl2 test is a non-destructive testing way. Testing methods are as follows:
1) First to clean the flexible pcb surface after developing, for example:
After developing → acid washing → acid etching → acid 
2) in a 5% Cucl2 solution for 30 seconds, the best to swing liquid slightly, and use a sponge to remove small air bubbles on the board to invoid to affect judgments.
3) washed
4) drying
5) visual inspection
The copper surface of normal developing will form a layer of gray and black oxide layer after handling.  When the  surface of flex pcb with dry film glue, there will still maintain a bright copper color. This method can be tested if developing of flexible circuits is good.

Related Posts:
1.Drilling of flexible circuits;
2.FPC Exposed Process Operation Notes.