It can be challenging to determine whether the tips on your pipette are contaminated, particularly because contamination often occurs at the microscopic level. However, the general warning signs listed below could suggest contamination:

• Visible contamination: If you notice any visible particles, fibers, or discoloration inside the pipette tip, it is probably contaminated. Throw away the old tip immediately and use a fresh one.

• Erratic pipetting performance: Inconsistent or erroneous pipetting results could be a sign of contaminated pipette tips. Careless pipetting can result in inconsistent liquid dispensing volumes, which can affect the results of your experiment.

• Unexplained experimental results: If your experiments are producing unexpected or inexplicable results, it is worthwhile to look into the possibility of pipette tip contamination. Cross-contamination of samples can hinder your research and lead to inaccurate results.

• Unusual liquid behavior: Unusual liquid behavior could indicate contamination, such as liquid sticking to the tip or droplets forming inside. These issues could contaminate your samples and reduce the accuracy of your pipetting.

• Reused pipette tips: It is best to avoid reusing pipette tips as this increases the risk of contamination. Even though you might think the tips are clean, there could be microscopic impurities that affect your experiments.

If you suspect contamination, it's always best to err on the side of caution and use a fresh pipette tip. This will help to ensure that the results of your experiments remain accurate and reliable. It's also critical to use the proper pipetting techniques and maintain a clean working environment to lower the risk of contamination.