It is not possible to measure MWCO without testing sample with specific solute at specific pressure. Because MWCO is mainly depend on the pore size and pore shape. The pore size and pore shape is largely depend on the polymer concentration ,miscibility between solvent and non-solvent during phase inversion, gel bath temperature and pore forming agent. So, you can measure the pore size of the membrane by SEM or porometry and then you can estimate MWCO of the membrane. This will minimize your experiments.

By AFM and SEM

If the objective is just to see the relative separation performance, it's possible to use a different set of polymer markers such as PVP. But if the interest is simply the size of the pores, SEM should also work. The presence of oversized defects could be detected by either staining or a high pressure air diffusion test using a low surfcae tension fluid like alcohol.