How can I find exactly what my codesign identity is?

In Xcode, you can access the Code Signing Identity by selecting your target, going to build settings and it's right under "Code Signing Identity".

For more information, you can access Apple's Documentation.

You can run following command in your terminal to get a list of all codesign identities installed on your system:

security find-identity -v -p codesigning

The output will contain lines like:

1) 482348ADF834384843884934734 "iPhone Developer: John Smith (YTHGJFKTHG)"
2) 49u343943943943494387348738 ...

where the first part (482348ADF834384843884934734) is your codesign identity, which is passed as an argument to the --sign flag of codesign tool.