It's pretty clear the vast majority of the time. If it's amongst other measures like quart and pint, it probably means half-pint. If it's amongst measures like glass and pinch, it probably means whatever a coffee cup might hold (usually 6oz, or 3/4 of a cup, or 3/8 of a pint). However, if the measurements are that inaccurate anyway, it's unlikely that the difference between 6oz and 8oz is going to make a lot of difference.

"Ounce" is a bigger problem, as it measures both volume and weight. It tends to be avoided though, most recipes skipping volume units from tablespoon to cup and not using weight measurements at all (unlike most European recipes).