As Alaa Kareem Niamah has mentioned, the method of measuring enzyme activity in SSF depends on type of enzyme. I have attached some books and article which the methods and effective parameters on enzyme activity in SSF have been explained. I hope they would be useful.
You have to add the desired amount (generally 100 ml for 4 gr substrate or 5 times) of acetate buffer (0.05 M) to the fermented material and incubate at 160 rpm for 1 hour. Then test the enzyme using DNS.