could try growth at differential temperatures. Pseudomonas aeruginosa should grow at 37-42 degrees C but Pseudomonas fluorescens should not. You could use CN agar which is more selective for Pseudomonas aeruginosa. Alternatively Pseudomonas aeruginosa should be resistant to kanamycin. However, a PCR specific for either organism would be a better approach. Or MALDI-ToF analysis. Or 16s?