Aside from estimating it, you can also ask the eMMC itself to tell you how far it has degraded so far (assuming it supports EMMC 5.1 or above).
The JEDEC standard (JESD84-B51) specifies that in the ext_csd field (offset 268-269) bytes indicate the level of type A/B wear. (value 0x1-0xA reflects 10%-100%, 0x0B means lifetime exceeded)
For example, on a typical linux distro, you could run the following script in bash to tell you:
RES=`cat /sys/kernel/debug/mmc1/mmc1:0001/ext_csd`
typea="${RES:536:2}" ;
typeb="${RES:538:2}" ;
typead=`echo "ibase=16; $typea"|bc`
typebd=`echo "ibase=16; $typeb"|bc`
echo "Type A percent: $((typead * 10)) %"
echo "Type B percent: $((typebd * 10)) %"