A flex circuit may have test structures and antenna structures. The test structures may include test capacitors and transmission lines. The performance of the test structures may be measured using test equipment. Pass/fail criteria may be applied to the flex circuit based on the measured values. If the flex circuit is a failing circuit, flex circuit manufacturing settings may be adjusted. The performance of a radio-frequency (RF) cable may also be measured using the test equipment. Sample portions of the RF cable may be obtained and measured. Pass/fail criteria may be applied to the RF cable based on measured cable loss values. If the RF cable is a failing cable, RF cable manufacturing settings may be adjusted. Antenna structures associated with passing flex circuits and RF cable segments associated with passing sample RF cable segments may be incorporated into a wireless device during production device assembly.