This is part of the problem that President Trump has been bringing up. Once a vote has been cast, there is usually no way of determining who the individual has voted for. This is by design in order to protect the privacy of the secret ballot. There would be records to show who voted and the information can be cross checked. However, many politicians (usually on the left) fight any attempt to cross check.
As an example there have been studies proving that illegal votes have been cast. For example, there have been areas that counted more votes than the total number of voters registered. I know people who have been unable to get their names removed from the voter roles after moving away from a jurisdiction.
I am citing articles that show that votes were miscounted. The fact that the articles also state how those votes were divided is not really relevant to the question. However, this is what the citations say so I need to quote them accurately even though that part of it may not apply.
The way the illegal votes are divided may indicate why some politicians are attempting to allow illegal votes to be counted, but that is a different subject for a different question.
It should be noted that Jesse Richman states that his article was only to object to the number of 3 - 5 million put out by others. However, the point of pointing to his research is to say that given the numbers that he uses it is still a significant number and would require sufficient research to prove or disprove his numbers. That is the reason that I reference his study. Whether the number is 8,000 80,000, 800,000 or 8,000,000 or any specific figure, it is something that needs to be corrected.
When there was an attempt to recount in Michigan, it was found that more votes were cast than there were registered voters.
Detroit Machines Registered More Votes Than Voters: Report
Voting machines in over one-third of Detroit precincts registered more
ballots cast in the Nov. 8 presidential election than the number of
voters tallied by poll workers, the Detroit News reported Tuesday.
Reports from the office of Wayne County Clerk Cathy Garrett revealed
optical scanners registered more ballots cast than the number of
voters recorded in poll books in 37 percent of Detroit precincts.
Trump argument bolstered: Clinton could have received 800,000 votes from noncitizens, study finds
Hillary Clinton garnered more than 800,000 votes from noncitizens on
Nov. 8, an approximation far short of President Trump’s estimate of up
to 5 million illegal voters but supportive of his charges of fraud.
Political scientist Jesse Richman of Old Dominion University in
Norfolk, Virginia, has worked with colleagues to produce
groundbreaking research on noncitizen voting, and this week he posted
a blog in response to Mr. Trump’s assertion.
Based on national polling by a consortium of universities, a report by
Mr. Richman said 6.4 percent of the estimated 20 million adult
noncitizens in the U.S. voted in November. He extrapolated that that
percentage would have added 834,381 net votes for Mrs. Clinton, who
received about 2.8 million more votes than Mr. Trump.