The invention relates to a method for testing the functionality of an FMCW-based fill state measuring device, which is used to measure the fill state of a content (2) that can be found in a container(1), and to a fill state measuring device, which is suitable for carrying out the method. In order to test the functionality, a microwave signal (S2) is generated, the frequency change (f'2) of whichdiffers from the frequency change (f'1) of the measuring signal (S1) that is used during the regular measuring operation. By comparing the frequency (f2) of the differential signal (ZF2) resulting from the microwave signal (S2) with a specified reference frequency (fp), the functionality of the fill state measuring device is determined. Thus, the fill state measuring device autonomously detects whether it is functional or whether an error has occurred, which has been caused presumably by device-internal interference signals. In particular, the invention offers a clear advantage with respect tomaintaining field device-related safety standards.