Ultrasonic flow meters are accurate and non-intrusive. However, one of their drawbacks is their sensitivity to installation effects especially when elbows or pulsating flows are found in front of the meter's inlet. Depending on the flow rate, such installations can create perturbations on the flow measurement. These perturbations cause an increase of the noise level as well as a possible error on the flow estimation. A technique using the noise level as a criterion for making a diagnostic of the error is presented. The perturbations are examined and compared to gaussian distributed signals. A basic method for detecting gaussian perturbations with equal means is described. The likelihood ratio is first computed in the general case. It is also computed making the assumption of uncorrelated signals. Receiver Operating Characteristics (R.O.C.) are plotted. The assumption of gaussian perturbations is then investigated. Optimal thresholds are proposed for both methods and for a large interval of flow rates (from 0.4 ml/s to 0,75 1/s). Probabilities of detection and of false alarm are evaluated for both methods. It is concluded that correct detection can be performed. The optimal threshold is to be chosen by the operator.