In the talk, we consider models of viscous incompressible fluids and discuss how the quality of their approximate solutions can be verified. For such problems as Stokes, Oseen, Bingham, it is shown how to derive guaranteed bounds of the difference between the exact solution and any approximation from the admissible (energy) space. If the latter space is is understood as the space of functions that not necessarily satisfy the divergence--free condition, then the derivation of the respective estimates is based on the LBB condition.