If you have no idea: Proteomics using Mass spectrometry
Mass spectrometry is a powerful
method that (in theory) identifies every proteins in a given solution. However, I don‘t know how robust this method is in terms of sensitivity in the presence of large background signals. Usually, people pre-process the sample, e.g. they separate the raw protein extract in a 2D PAGE (polyacrylamid gel electrophoresis), separating the proteins by size and isoelectric point in a gel. Then they cut the gel into pieces, extract specific proteins and then identify protein fractions using mass spectrometry.
Advanced mass spectrometry methods seem to be able to identify proteomes from imaged tissue samples Ryan et al. 2019
A pulldown assay is also powerful method to pre-isolate unknown components of a protein complex. But for that you need one component or an antibody that binds to a component of the protein complex.