HeadCount is an organization that attempts to help individuals to register to vote, and also to help them know what identification is needed at the polls. The web page is especially geared to young people. Here is a link to a particularly helpful chart that shows whether student ID cards can be used at the polls. The web page also has a chart showing the deadline for voters to register in each state.
The simple solution to this is to ink the index finger/thumb of the voter once they have completed their ballot. Semi-permanent ink that takes several days to wash off. Eliminate all early and mail-in voting and only permit absentee voting for military and others with legitimate written statement at the county courthouse. If it’s good enough for Iraq and Afghanistan, it ought to be good enough for us.