If I am given the ER Diagram with the describing text, then I can simply write the corresponding relations. But how could the corresponding Normal Form be specified?
The steps to figure out the normal form of a relation are:
You need 1. to work out 2., and 2. to work out 3. If you are given only the ER diagram, you have to figure out the functional dependencies yourself trying to understand the relationships between the different attributes.
Also, note that there isn't an algorithmic way to compute which the FDs are since it very much depends on your domain, so you rely on your common sense.