How to tell if BBB is booted to the eMMC or the microSD card if both has the same version of Debian?

As above subject

Its easy if the sdcard and MMC have different sizes.

There are a few other things I haven/'t verified – I suppose you can look in dmesg – on my sdcard debian 9 host I’m seeing:

[ 4.680486] mmc0: new high speed SDHC card at address 1234
[ 4.686231] sdhci-pltfm: SDHCI platform and OF driver helper
[ 4.692167] ledtrig-cpu: registered to indicate activity on CPUs
[ 4.698939] mmcblk0: mmc0:1234 SA08G 7.21 GiB

[ 4.822750] mmc1: new high speed MMC card at address 0001
[ 4.828919] mmcblk1: mmc1:0001 S10004 3.56 GiB
[ 4.833762] mmcblk1boot0: mmc1:0001 S10004 partition 1 4.00 MiB
[ 4.839948] mmcblk1boot1: mmc1:0001 S10004 partition 2 4.00 MiB
[ 4.847235] mmcblk1: p1

I’m interested too in a “definitive” (not heurestic) method.