Build your design. Then look at the Design-Wide Resources Pin Editor. The table will show you where PSoC Creator mapped the output.
You can see the mapping of the opamp by referreing to the cyfitter_cfg.c file in the Workspace. In AnalogSetDefault(void), the opamp used will be specified.
Another way of determining it is as follows, In case you use an opamp but did not assign any pin in the .cydwr Pins tab, then Creator will assign one of the pins P0, P0, P3 or P3. But looking at which pins has been chosen, you can figure out which opamp is used.
There is also provision to Force Creator to use a Specific Analog Block. Like say, in your design you want to use opamp0, then you can do so by going to 'Directives' tab in .cydwr, choose Directive Type as 'ForceComponentFixed' and force Creator to use opamp0.
Excellent, thanks very much folks.