Good note. <pedantic>Though, I'd still write it as
if mountpoint -q $1; then
echo "$1 is a mountpoint"
echo "$1 is not a mountpoint"
It's a little easier to type, a little cleaner to read and, if you end up inserting lines in a complex script, you don't have to worry about losing track of the value of $?, which is volatile.
Also, technically, when using the old style test, it should be a single '=' (for string comparison) or '-eq' for integer comparison rather than the double '==' (ash, for instance, will throw an 'unexpected operator' error). And, if using bash as our shebang declares, it should use the new style test, anyway. Just making the command's exit status the conditional avoids the historical baggage of the tests.</pedantic>
Still, the essence of it is that mountpoint
, if available, would definitely have been the command to use.