I think that discharge does increase a bit in the last trimester. It can be the mucus plug or just increased CM.
If you smell it, amniotic fluid has a sweetish and fairly distinctive smell (some say its a bit like semen).
There is a test they can do fairly quickly at the hospital to check if its aminiotic fluid or not so maybe you could ring up and arrnge to get it checked if you're worried.
I think a scan would reveal if your levels of amniotic fluid were too low but maybe not show a rupture in the membrane.
If you can't use normal liners have you considered using cloth sanitary products? There is athread about them here. It might save you from having to wear damp nickers.