Does "covalent" AgCl (aq) exist and if so, how to quantify its concentration?
Some amount of $[\ce{AgCl(H2O)}]$ particles should exist in solution from pure thermodynamic point of view. See, $\ce{Ag+}$ ion can coordinate two ligands, and in solution both $\ce{Cl}$ and $\ce{H2O}$ are present. However, it should be really hard to detect trace amounts of this particle, as $\ce{AgCl}$ is almost insoluble. In case one can get concentrated salt solutions, similar particles may exist and hints for their existence may be got from conductivity measurements and osmotic pressure. Sometimes a direct study using NMR is possible, which can give some structural information if used properly (see 2D-NMR methods). As I remember, $\ce{Cr(CH_3COO)2}$ hydrate is slightly soluble in water but undergoes little to no dissociation.