Firts of all, if the bit is somewhere at all, it's never in the software, but in the PLC.
Now, if you're talking S7-300/400 then there's no such bit. Instead you have a block called at startup. Depending on the type of CPU you're using and the type of startup you're trying to detect, you can use OB100/101/102.
If you're talking about S7-200, then you can use SM0.1.
Kind regards,