Electron spin resonance (ESR) was measured on high-mobility two-dimensional electron gases (2DEG) in modulation-doped SiGe/Si quantum wells. We observe a resonance line as narrow as 40 mG, which allows the detection of as few as 10(9) spins in a standard microwave absorption configuration. Longitudinal spin relaxation times T-i of up to 30 mu s were found, which is six to seven orders of magnitude longer than the momentum relaxation times in these samples. Using either gated samples or samples where the 2D carrier density n(s) can be adjusted persistently by illumination, the ESR signal was recorded as a function of the carrier density. By measuring the integrated ESR absorption the density of states (DOS) at the Fermi level can be derived as a function of n(s). The 2D DOS shows a tail, which allows a quantitative extraction of the potential fluctuations.