Hi – the high pH in the spirulina pond usually prevent that other algae colonize the pond and if you have a commercial farm you should have the spirulina culture tested every other month by a lab and check it almost daily in your microscope.
An excellent way to check for bacteria is to “Gram Stain” a sample on a slide for the microscope. A gram stain makes bacteria visible on a slide when they are normally nearly invisible. Gram staining is outlined on youtube and protocols easily accessible on the internet. It can be done in 5 minutes and $20 for a huge supply of stains. Amazon has the stain and a microscope that sees 100x 400x and 1000x for 84 bucks. Learn how to use immersion oil for 1000x. but anyone can stain and look for bacteria at 400x easy.