command line

to find out what bootloader you're using, you have to look at the bootloader!

try dd if=/dev/sda bs=512 count=1 2> /dev/null | grep -q GRUB && echo "GRUB found"

All querying of installed packages, as described in the other answers, miss an important point: to probe your bootloader, look at the bootloader!

In other words, you can install whatever packages you like in whatever OS or distro you like, but none of this has anything to do with the bootloader you'll use when you reboot.

specifically, for MBR/msdos partitioned disks: probe the MBR (i.e. look at the bootloader to see what bootloader you're using)

This duplicate has some good answers: How do I find out which boot loader I have?

I like this more complete answer, found in a newer question: question: https://superuser.com/questions/466086/how-can-i-discover-which-bootloader-is-installed-where answer: https://superuser.com/a/466248