如何知道系统目前正在安装ISO ?

I am creating an RPM. This RPM will be packed with the CentOS ISO and will install when CentOs is installed. This can also be installed using normal rpm installation method. But rpm installation need to behave differently if it is installed from ISO and installed by rpm normal installation method. So, the question is how do I know whether the system is currently installing the ISO. I am creating an RPM. This RPM will be packed w