I don’t see how you’ll ever be able to do this. Even if the number you get is 0.5, you don’tknow the user didn’t enter 0.5000000000000000000234. And if you had a quad precision number, you wouldn’t know if the user entered 0.5000000000000000000000000000000000000000234.
I don’t know how much control you have over this, but can you make the user enter two values, one for the integer portion, and one for the fractional portion?
e.g. on a form, you could have two boxes side-by-side, separated by a .
and then you could check how large the fractional portion was
e.g. the user would see something like:
Enetr value: [ 999].[99 ]