You can enable verbose debugging for the git command by doing:
export GIT_TRACE=true

Since git is using ssh to make the connection, it will be ssh that is using the keys.
Normally ssh will try these keys by default:

~/.ssh/id_rsa
~/.ssh/id_dsa
~/.ssh/identity

in that order.

To specify a different key for your git connection use an entry in your ~/.ssh/config file for the git hosts.
As an example, with springloops the git host is:

ssh://[email protected]:1234/

thus your entry will be

host slsapp.com
user sls
hostname slsapp.com
port 1234
identityfile ~/.ssh/id_dsa