In other words, there is no way to find out what pfile was used to start the instance. It is possible even that the file was altered or doesn't exist anymore.

In [oracle_home]\database, i've a file called init[sid].ora which redirect to another init[sid].ora file..... but when i verify parameters in this file, they doesn't correspond with parameters in my database.... very strange...
Not so strange... maybe someone changed the pfile without bouncing the database, so none of the changes took effect on the instance.