Having little beasts growing in your cultures is not a good thing – after all, you are studying your cells, not whatever hosts decide to join them… Depending on the contamination, you will observe different anomalies in your culture:

• Macroscopically: if your medium looks yellow or very cloudy (notice the difference between high-cell-density-cloudiness and contamination-cloudiness; you should see that under the microscope)
• Microscopically: if you see cells that don’t look like yours, or funny little worms moving around, or aggregates of very small thingies – you have a problem
• Analytically: because some species are too small (<1μm) to detect under the microscope, there are analytical techniques to test their presence. These include DNA staining, PCR or agar plating (to amplify the numbers).