If you want to test if your protein is secreted, check for its presence in the LB culture medium by ELISA, Western blot, or an enzymatic assay (if applicable). You don't necessarily need a control protein for this purpose. Secreted proteins typically have a leader sequence present. If you can demonstrated that deletion of the sequence sequesters your protein intracellularly, I think that will be sufficiently convincing of specificity of your measurements.

Dear Daniel,

Thank you for your informative answer. More precisely, I want to assess the cell lysis during bacterial growth as next to the proteins possessing signal sequence, I find typically intracellular. Would LDH be a good target? No antibodies are available.

Agata- why do expect any lysis at all? If there is an easy and sensitive assay for LDH, that would seem like a reasonable choice. I don't have experience targeting endogenous E. coli proteins so can't recommend an alternative. Many people would choose a transgene such as CAT or luciferase for the purposes of generating a reporter.