Find bootloader

I don't believe this info is tracked in meaningful way under either /sys or /proc. About the only way I can fathom this would be accessible to you after a boot is by interrogating the system either by looking to see if a GRUB or Lilo configuration file was present, or by making use of a script such as bootinfoscript.

Example - check boot device

If you know which device your system was booted with you can use dd to dump the contents of the boot loader and then grep for either GRUB or LILO.

                      

You can use these commands to determine whether you're using GRUB or LILO:

$ sudo dd if=/dev/sda bs=512 count=1 2>&1 | grep GRUB
$ sudo dd if=/dev/sda bs=512 count=1 2>&1 | grep LILO

Which ever returns this string is the boot loader you're using:

Binary file (standard input) matches

Example - using bootinfoscript

$ sudo ./bootinfoscript --stdout
                  Boot Info Script 0.61      [1 April 2012]


============================= Boot Info Summary: ===============================

 => Grub2 (v1.99) is installed in the MBR of /dev/sda and looks at sector 1 of 
    the same hard drive for core.img, but core.img can not be found at this 
    location.

sda1: __________________________________________________________________________

    File system:       ext4
    Boot sector type:  -
    Boot sector info: 
    Operating System:  
    Boot files:        /grub2/grub.cfg
    ...

Neither of these approaches is "conclusive", however, since multiple boot loaders can coexist, but at least it gives you a rough idea of the boot loaders that "might" be in use.

References

• How do I find out which boot loader I have?