Each fluid manufacturer generally produces documentation on their fluids.
A fluids concentration, derives its LOUT or lowest operational use temperature. For Type I this is the fluids freezing point to which 10°C is added.
The freezing points are concentration specific up to a point but above approx 75%-80% concentration the glycol in the fluid concentrate is such that it does not freeze.
While this seems like an ideal scenario the fluids viscosity behaviour changes and can become a sticky substance that will not eliminate as designed during the take off if these temperatures are encountered.
The problem that then arises is when concentrated type I is mixed with water to acceptable ratios when will it freeze?
As the other contributors have stated this can be difficult to assess. Because Type I fluids are by their design low viscosity (runny) the fluid layer is usually very thin.
This thin layer couple with the fact a certain amount of evaporation and transpiration is taken place means Type I fluids have a tendency to fast freeze and require close observation.