A novel detection scheme for detection of liquid levels and bubbles in microfluidic systems, using the principle of total internal reflection (TIR) is presented. A laser beam impinges on the side walls of a channel which are inclined at 45deg. In an unfilled channel of such a "V-groove", TIR deflects the beam by 90deg into a simple light detector. Upon the presence of liquid, the refractive index in the channel changes, thus eliminating deflection by TIR. The detection principle is robust, requiring no calibration, and tolerating alignment errors of the laser larger than the width and depth of the microfluidic channels. The machining of the V-groves can seamlessly be integrated into common polymer microfabrication schemes such as injection molding.