We present a noninterferometric method for direct and complete characterization of the degree of first-order temporal coherence $\gamma \left(\tau \right)$ of low-coherence broadband sources both in amplitude and phase. The method is based on measuring the radio-frequency transfer function $H_{\mathrm{RF}}\left(f\right)$ of amplitude-modulated partially coherent guided waves in a system with first-order dispersion. Experimental data for sources based on amplitude-modulated amplified spontaneous emission in the $1.55\mu \mathrm{m}$ band having spectral widths up to $30\mathrm{nm}$ and subsequently dispersed in a $12.77\mathrm{km}$ standard single-mode fiber coil provide complete characterization of the temporal coherence with $6.5\mathrm{fs}$ resolution and a dynamic range exceeding $20\mathrm{dB}$ .