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I don't see a "quick and dirty" answer, so I'll post my solution:

One line

openssl recent enough

userline=$(sudo awk -v u=$user -F: 'u==$1 {print $2}' /etc/shadow); IFS='$'; a=($userline); [[ "$(printf "${pass}"|openssl passwd -"${a[1]}" -salt "${a[2]}" -stdin)" = "${userline}" ]]

with perl

user="user1";pass="eeeeee";userline="$(awk -v u="$user" -F: 'u==$1 {print $2}' /etc/shadow)"; a="$(echo "$userline"|grep -Eo '^\$.*\$.*\$')"; [[ "$(perl -e "print crypt('${pass}', '${a}')")" = "${userline}" ]]

In both case, it's a one-liner that returns 0 if the supplied password is correct.

You need three things:

1. Set the variable "$user"
2. Ensure that the user exists in the /etc/shadow (e.g., if ! grep -q $user /etc/shadow; then return 1; fi)
3. Set the variable "$pass"

Some explanations

1. First get the shadow line of the user
2. Split it on $
3. Use the openssl command to generate the string from the supplied password
4. Check if the generated string matches the stored one