How could you confirm that a protein is produced by pathogenic E. coli strains

If you take a gene sequence from pathogenic E. coli and run it through BLASTX to find that it has a match for an outer membrane autotransporter protein, how could you confirm that this protein is produced by pathogenic E. coli strains?

To confirm that a protein is expressed, you could do a protein gel, transfer the proteins to a Western Blot, and incubate with an antibody against that protein.

Or, a slightly less solid proof, you could show that the gene is expressed by doing a reverse transcriptase PCR (RT-PCR) on cDNA, so a specific PCR that proves that that particular gene is transcribed into mRNA. It does not prove that protein is produced, but you could at least show that mRNA is made.