Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency care if you have signs of severe dehydration or a serious complication, including:
Arrange urgent medical assessment for bloody diarrhea, a high fever, severe or worsening abdominal pain, repeated vomiting, or black tar-like stool.
Bismuth medicines can harmlessly darken stool, but sticky, tar-like stool may indicate bleeding.
And for anyone, someone who:
The article also says
Get medical advice if diarrhea is not improving after several days, continues beyond two weeks, or returns after seeming to clear.
Also seek advice for weight loss, ongoing greasy stools, signs of dehydration, or symptoms beginning after antibiotics.
Children, older adults, pregnant people, and those with weakened immune systems, kidney disease, inflammatory bowel disease, or other significant health conditions should seek help earlier.
When requesting care, mention recent travel, destinations visited, and any medicines or antibiotics taken.
Every sign above is quoted from the sofidex article on Traveler's diarrhea. If you are not sure, err upward — it is always reasonable to be checked.