Query for which my.ini in use
It is not possible to do that, because once the config is loaded on startup, it is no longer used.

MySQL uses a deterministic algorithm for searching for a config file. You can see it on the MySQL Manual (the first table is for Windows).

Two things you can do is: check which options are loaded by default: use --print-defaults:

mysqld --print-defaults
mysqld would have been started with the following arguments:
--log_bin


(in this case, I have log_bin in my configuration file).

Force manually a configuration file to be loaded with the parameter --defaults-file:

mysqld --defaults-file=C:\my\path\to\my.ini