You can look at the reflected signal from a smooth, flat surface. Move the transducer closer or further from the surface to maximize the amplitude. Or better yet, if you can display the spectrum, maximize the area under the spectral curve. Maximized the amplitude of the RF should be fairly good. Once you find the right distance, you can measure it from the round trip time starting at the trigger to the waveform.

NICE idea by kyle W Hollman--- i think its easy to move the 1-3 focus arrow towards the area of interest --