You're almost right. Over a long period of time, the gas evolved will be the same. (Since even though the acid may be weak, as hydrogen gas is created it will push the equillibrium over to the side of the dissociated acid, thus causing more acid to dissociate). So the best thing to do would be to use a large amount of acid and metal and set a time limit, say one minute, for the amount of hydrogen gas evolved. The more gas evolved in that time frame, the stronger the acid is.