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.