Simple test: take a clean glass full, hold it up to strong white light with the glass between your eyeballs and the light. If its 'hazy' refilter it a few times ... until its clear and with NO haze.
If there is NO or very little 'haze' its still as good as the day it was 'cracked' ... and then simply say a prayer to your 'cosmic muffin', on behalf of your 'stability additive' manufacturer etc. that you dont have a fungal, etc. infection in your tank.