jack_delay is a small command line JACK app you can use to measure the
latency of your sound card. It uses a phase measurements on a set of
tones to measure the delay from the output to the input. Accuracy is
about 1/1000 of a sample.

This requires jack-audio-connection-kit, version 0.120 or greater (the
one on SlackBuilds.org is fine, but if you got your JACK from a different
source, make sure it's new enough, or replace it with the SBo version).

Definitely read the docs in /usr/doc/jack_delay-$VERSION/README to
understand how to use this.