  We report detailed magnetometry and high-frequency electron spin resonance
(HF-ESR) measurements which allow detailed investigation on Li-Fe antisite
disorder in single-crystalline LiFePO$_4$, i.e., exchange of Fe$^{2+}$- and
Li$^+$-ions. The data imply that magnetic moments of Fe$^{2+}$-ions at
Li-positions do not participate in long-range antiferromagnetic order in
LiFePO$_4$ but form quasi-free moments. Anisotropy axes of the magnetic moments
at antisite defects are attached to the main crystallographic directions. The
local character of these moments is confirmed by associated linear resonance
branches detected by HF-ESR studies. Magnetic anisotropy shows up in
significant zero-field splittings of $\Delta = 220(3)$~GHz, $\Delta'\sim50$~GHz
and a highly anisotropic $g$-factor, i.e., $g_\mathrm{a} = 1.4$, $g_\mathrm{b}
= 2.0$, and $g_\mathrm{c} = 6.3$. We demonstrate a general method to precisely
determine Fe-antisite disorder in LiFePO$_4$ from magnetic studies which
implies a density of paramagnetic Fe$^{2+}$-ions at Li-positions of 0.53\%.
