Distinguishing between stain and flow can be more art than science. When you see blood emerge from your body, that is a flow. When you have bleeding consistent with a light period day, that is usually considered a flow. Needing to change a liner every few hours is also usually a good rule of thumb for identifying a flow (though some women are more particular about changing than others).
It bears noting that blood found on a tampon typically makes a woman niddah even if she is only staining. (So we usually do not recommend tampon use unless a woman is already in niddah.)
We hope the above helps somewhat with clearing up the confusion. You can always get in touch with us here or on our hotline with a more specific question, and a conversation on our hotline or with a local yoetzet might be helpful.
Please write back with any follow up questions.