See the last entry at http://www.istechsupport.com/faq.php.

One way, is in EVO maintain database if you pull up a given file, it lists in the "Sort" field the different fields that are indexed. you'd have to assume the starting order is 0, 1, 2, etc.
 
another way, is to open up DBA Classic, and go into Maintain Database there. select the file you are intending on rebuilding, and you will see the Indexed fields with their corresponding numbers.
 
needless to say, the key number is what you are refering to as which indexed field.  
 
for all practical purposes you will use the key number 0 unless you want to have the records sorted during the rebuild on a different indexed field.
 
i didn't list the option that if you have an ODBC connection to the database, open the PCC and connect to that db, you can visit the properties of the table and see the indexes too. it should match up accordingly, however be aware that you are looking at different dictionary files than the one EVO uses. they technically are the same, base on last time they were created..... but i didn't want to muddy the waters too much.
 
 
ok, HTH.