How to determine if psql returns ZERO to the "shell" and finished successfully?

Hello,

I'm implementing a Setup Wizard to install PostgreSQL. In the documentation
it states that psql returns ZERO to the "shell" if it finished successfully.

I'm implementing a wizard in Pascal and was wondering the correct syntax to
check whether or not the shell received ZERO?
For example,

If not (psql) then
MsgBox('Note: psql was not finished normally.', mbError, MB_OK);

How exactly can I check to see if psql returns ZERO to the "shell"? What do
I test for? If(Psql.exe)? or is there a variable I can check to see if it
has been set?

Any suggestions would surely be appreciated. Thanks.