Do you mean, "How will we know if a ratio is in lowest term?"
If that is what you mean, then this is my answer:
We will know that a ratio is in its lowest term if the numbers cannot be divided by a common factor anymore except 1 (which would give the same ratio after dividing ) or maybe, we will know if the ratio is in its lowest term simply if the numbers do not have any other common factor except one. ( maybe, it is as simple as that?)