Low probability of intercept (LPI) radar systems is designed "to see and not to be seen." Low peak power, low sidelobe antenna pattern, wide bandwidth, and spread spectrum waveform are their typical characteristics to prevent detection by noncooperative electronic warfare receivers. As a result, in terms of Electronic Warfare systems, the detection of an unknown LPI radar has great importance and requires special techniques. While a few studies have been done on this subject, many open points still need to be managed. This paper presents an LPI radar signal detection algorithm based on autocorrelation and time-frequency image-based moments. Detection does not require any prior information on radar signals. Detection performance for different types of LPI radar signal waveform is studied. The results showed that detection is possible under low SNR.