The control of EGNOS Ranging and Integrity Monitoring Stations (RIMS) is a constant task in order to verify the correct functioning of the RIMS network, and to detect or identify possible receiver malfunctioning. Each EGNOS station currently uses a minimum of two independent RIMS, to ensure a constant back up. The availability of different observables and quality parameters from these two receivers allows studying in details potential observed disturbances, to detect unnoticed behaviours, or to investigate accidents. Among the different tasks, the study of the quality of the code and phase measurements is very important to quantify any problems affecting the tracking process. The errors affecting measurements are well known: unmodelled satellite clock and motion, receiver clock (static receiver), multipath, thermal noise, atmospheric delays, interference. RIMS code measurements are dominated by noise and multipath, while phase measurement will be more sensitive to smaller range variations such as the satellite and receiver clock, or ionosphere activity. Thus, a different analysis tool is necessary in these two cases. Because carrier-phase measurements are much more sensitive (in relative magnitude) to most of the sources of errors than to tracking errors, it is also more difficult to observe phase lock loop tracking errors compared to the code tracking case. It is then understandable that phase measurement quality assessment is more difficult. The manipulation of L1 and L2 measurements allows the cancellation or reduction of most of the non-tracking errors, but it might not allow isolating a specific error. Thus, it is preferable to work on each easurement separately. The goal of this paper is to present a methodology to assess code and carrier-phase measurement quality, as well as to show the tools used to define this methodology. The first part of the paper concentrates on the use of polynomial interpolation to model the measurements and thus study the corresponding residuals for quality control. It is seen how this method is suitable for pseudoranges, but not for phase measurements due to the aforementioned reasons. To understand exactly the errors preventing the use of polynomial interpolation for phase measurements, the second section of the paper shows a tool that was realized to allow the visualization of measurements variations over time. This is obtained by using models for most of the known components of the transmission time. This tool is particularly interesting to have a visual access to the residual phase error variations of different RIMS. It can then be used to isolate specific local errors, or to understand the source of these residuals, such as satellite clock variations, excited ionosphere, or tracking problems.