The voltage multiplier (VM) is a principal component in high-precision digital-to-analog (D/A) converters based on rapid single flux quantum (RSFQ) circuits. Experimental confirmation of correct operation of the VM is important to guarantee the accuracy of the D/A converter. In this study, a circuit for direct comparison of the output voltages of VMs was designed and demonstrated. An on-chip SQUID sensor was utilized to detect the VM errors. Bit error rates of a single-stage VM were successfully measured down to 10(-12) in the constant-voltage regions. A method of detecting errors during transients was also proposed and carried out.