Microfluidics has shown the ability to reproduce many of the techniques used in chemistry and biology, achieving faster and inexpensive results with smaller samples. As part of this effort towards miniaturization, the heating and cooling of samples has become an important step in many devices, especially for nucleic acid analysis. Measuring temperature profiles inside microchannels is required to verify the performance of such devices. Here we introduce a new technique to measure thermal profiles in microchannels: leuco-dye temperature sensitive microbeads, which permits fast, simple and inexpensive measurement of thermal profiles based on color, which enable its use to quickly assess temperature profiles to verify device performance, or as an educational tool for visualizing microscale heat transfer. Using the technique, we present visualization of thermal profiles and measurement of color transition lengths in microfluidic devices and compare the results with theoretical and numerical models with good agreement.