There is no obligation to become niddah on a regular basis. If you do not experience an actual flow, but rather have monthly episodes of staining, you may take precautions against becoming niddah by wearing colored underwear (or disposable pantyliners — even white ones) and waiting at least a few seconds (ideally 15 seconds) after urinating before wiping. We also recommend abstaining from intercourse until 24 hours after the staining subsides.
If you find a questionable stain (e.g., a stain larger than the size of a gris on a white garment or on your skin) you can bring it to a halachic authority for evaluation.
You can find a more extensive discussion on our page on stains.
Additionally, you should not perform a bedikah unless required (such as during the clean days or on an onah day) since stains found on internal bedikot are treated strictly.
Please feel free to get back to us with any further questions.
B’Hatzlacha!
This response has been updated to reflect the rulings of our current Rabbinic Supervisor, Rav Kenneth Auman, regarding waiting before wiping.