We'll use ssl-cert-check, so if it's not yet installed (I'm using Ubuntu):

sudo apt-get install ssl-cert-check

Then, locate your certificate and perform:

sudo ssl-cert-check -c /etc/letsencrypt/live/<domain name>/cert.pem

/etc/letsencrypt/live/<domain>/cert.pem

Host                                            Status       Expires      Days
----------------------------------------------- ------------ ------------ ----
FILE:/etc/letsencrypt/live/<domain>/cert.pem Valid        Feb 28 2018  65