It is not difficult to test the retro-reflectivity of a laid line, as the cost of the testing apparatus, a retrometer, is not that expensive, and their reliability is much improved compared to earlier models.

The difficulty and cost of the traffic management can be the main factor when conducting performance tests on laid road markings.

If you are able to conduct the retro-reflectivity testing at the time the road marking takes place, perhaps on a random basis, you will save on traffic management costs and be able to inform the contractor of the quality of the road markings he is, or is not, producing.
The random nature of the testing will mean that the contractor will not know if you will be turning up or not, and they are likely to make sure all markings are of the required standard.

Retro-reflectivity testing of newly laid thermoplastic road markings with surface applied glass beads, being performed at the time of laying to avoid traffic management costs.

An example of a retrometer for testing the retro-reflectivity of road markings, other makes are available, and the models now available are much improved in accuracy, reliability and ease of use to early models.
They can also be easily calibrated by performing tests on a reference panel supplied with the apparatus.