EDIT: my bad, I didn't read the KB thoroughly enough to realize it's basically useless as an answer on its own.
A more useful way to determine if you're in safe mode of not is from: Microsoft® Windows® Internals: Microsoft Windows ServerTM 2003, Windows XP, and Windows 2000 by Mark E. Russinovich, David A. Solomon.
The Windows kernel scans boot parameters in search of the safe-mode switches early during the boot and sets the internal variable InitSafeBootMode to a value that reflects the switches the kernel finds. The kernel writes the InitSafeBootMode value to the registry value HKLM\SYSTEM\CurrentControlSet\SafeBoot\Option\Option Value so that user-mode components, such as the SCM, can determine what boot mode the system is in.
Take the above and pair with the below, and you'll a have registry location you can check with a numerical value you can translate into something useful.
From the support.microsoft KB titled, "How to determine whether the system is running in Safe Mode from a device driver."
The Windows OS kernel exports a pointer to a ULONG variable that is
named InitSafeBootMode. This variable contains the Safe Mode settings.
A device driver can determine whether the system is running in Safe
Mode by the value of the InitSafeBootMode variable. A value of 0 means
that the system is not running in Safe Mode.
The following table lists the modes for other values.
*Note The value of 3 applies to Windows domain controllers only.