The high-frequency gravitational wave window above kHz offers the tantalising prospect of testing theories of beyond the Standard Model physics without having to contend with astrophysical foregrounds. This exciting opportunity has led to many proposals aimed at detecting these signals, often making use of existing infrastructure intended for wave-like dark matter searches. We will discuss how basic arguments can be made to estimate the sensitivity of most high-frequency gravitational wave detectors. Our findings will instruct the approach necessary to realise the goal of detecting new physics.