bubblestar

Use these commands in the Asterisk CLI for a detailed listing of the actual capabilities:

:> core show tanslation recalc 10
OR
:> core show translation
OR
:> core show codecs

Tha above are just for informational purposes.  It does not indicate anything about your configuration.

To tell which codec is being used for a specific call use one of the following CLI commands:

sip show channels
iax2 show channels

In general, you can define the codecs disallow or allow in specific extensions under sip.conf like this:

disallow=all
allow=alaw
allow=ulaw

bubblestar

As specified in my reply, you can use the command sip show channels to learn the active channels being used.  You will get something like the following format in the screen:

Peer             User/ANR    Call ID      Seq (Tx/Rx)  Lag      Jitter  Format
192.168.0.206    206         79010fe1-5a  00101/61141  00000ms  0000ms  ULAW


Yes, codec placement is in order where first available codec comes first.  
disallow=all
allow=alaw
allow=ulaw
allow=gsm
allow=g729

In above case, alaw will be chosen first.

However, disallow must be put before allow. Otherwise, you may have problem.