In the paper a method for estimation the circuit testability on the Register Transfer Level (RTL) is presented. The method allows to perform fast testability estimation in linear time complexity (regarding the number of components and interconnects of the circuit). Proposed approach is based on utilization of controllability and observability measurement for estimation of overall circuit testability. The application of developed method is demonstrated in a software tool for the development of RTL benchmark circuits with predefined testability properties. The results gained by our testability analysis method are compared with the results of professional ATPG tool. Experiments show the good correlation of the results obtained by our method and professional ATPG tool with significantly lower time complexity when our algorithm is used.