How does a GTM prefer a DC

Looking into some GTMm config we have for the first time....

We have a prod and non prod DC with a GTM in each.

The GTMs have a wide IP for a test site. Currently doing a NSLOOKUP on the test site resolves the production DC IP. If i disable the VIP on the LTM that the GTM is monitoring the NSLOOKUP will return the address in the non-prod DC. So all that is working well.

I just can't see where in the GTMs is there a configuration to say 'use prod DC as primary'

Can anyone point me in the right direction here?

Just need to know how the GTMs determine the best DC to use, and how when one DC goes down, the non-prod GTM takes over.

thanks