The Talmud brings a Mishnah: בָּצֵק הַחֵרֵשׁ, אִם יֵשׁ כַּיּוֹצֵא בּוֹ שֶׁהֶחְמִיץ — הֲרֵי זֶה אָסוּר. This refers to “deaf dough”, or dough like earthenware, so we cannot tell if it is leavened (hametz). If it is leavened it is forbidden on Passover, but since we do not know we try to compare it to another dough made at the same time, and look for signs of leavening in the other dough.
This is part of a larger concept of comparing one item to the group it comes from. Yes, there are always exceptions that prove the rule, but in general the rule is valid. When a certain group or generation displays similar traits, you can draw on that information to make assumptions about the individual from that group.
The Gemara notes than when there is no dough from the same batch to compare, we rely on an objective measurement, the time to walk a mil, which is reckoned at 18 minutes. In Jewish law, we assume that a dough left alone for 18 minutes will leaven, even without outward signs. Of course, with tell tale signs of leavening we know it is hametz. This leads us to another point: when you cannot rely on comparisons, look for objective measures. How does this person behave towards you? How about to other people? Be logical and evaluate people by their actual behavior.
Red Pill Rabbi 