How to decide which private key to use??

[Ramon F Herrera]

I am using the GPGme library to write a program that will
decrypt many files prorgamatically.  The files to be decrypted
have been encrypted with exactly one out of three keys.

When I decrypt those files interactively, gpg shows me what
key(s) was used and I type the passphrase corresponding to that
particular key.  The problem is, I am not sure how to replicate
that behavior within a program:

  - should I keep on trying our 3 corporate private keys
    (passphrases) until one of them works? (trial and error)

or:

  - is there any way to request the key from the file,
    in advance, and use the corresponding passphrase?

TIA,

-Ramon F Herrera