Before you embark on toxin quantification, you must first determine if the organism produces toxins & the type(s) of toxins produced by the fungi.

Literature search will help you narrow your suspicion & toxin screening analysis will take care of the rest.

Toxin Test

1. Aflatoxin

2. Ochratoxin

3. Fumonisin

4. Trichotecin

5. Zearalenone

For lower fungi, while for higher fungi like mushrooms you can test for

1. Amanitoxin

2. Muscarin etc.

I think this Article Qualitative and Quantitative Analysis of Mycotoxins

help you.

you can do the quantitative detection of mycotoxin produced by Alternaria brassicae bu using HPLC