>how are people supposed to know in advance of the election that they are voting for losers?

Because we have past data on voting patterns. We have countless examples of third party candidates failing to get even a single electoral vote. We also have plenty of survey data and mountains of other evidence that points to the fact that many voters just vote for whoever has the R next their name every time or whoever has the D next to their name every time. We do have evidence that there are voters who aren't fixed in their voting patterns, and who sometimes vote R, sometimes vote D, and sometimes vote third party; but these non-fixed voters do not represent a large enough proportion of voters to be able to elect a third party on their own. So, we KNOW with very high certainty that in order to win, you MUST win either the voters that only vote D or the voters that only vote R, while also capturing some of the voters that aren't fixed. Therefore, we can rightly conclude that any non-R or non-D candidate has virtually zero chance of winning; more surely than one's chances of winning the powerball.