Let me start by saying that I don't believe that election fraud has been demonstrated.
Glenn Greenwald and Matt Talibbi had an interesting discussion about it. Both are politically at the left but more anti-government power then most leftist these days. They point out that the US has a system that's build to be easy to manipulate. Attempts at changing the system by allow for fast counting of election results and making it harder to manipulate by Tulsi Gabbard got blocked. The US isn't a third-world country and could afford to have a decent election system. When proposals to build such a thing get blocked you have to ask yourself whether they get blocked because powerful forces benefit from not having a decent election system.
Claims about voting fraud being non existent and thus shouldn't a concern when talking about mail-in-ballots can reasonably trigger bullshit detectors.
Quote from the link from waveman's answer:
This fact has been attested to by American media on both the right and left. For instance, in 2012 the New York Times wrote the following: “While fraud in voting by mail is far less common than innocent errors, it is vastly more prevalent than the in-person voting fraud that has attracted far more attention, election administrators say. In Florida, absentee-ballot scandals seem to arrive like clockwork around election time.” – Liptak (2012)
The fact that the New York Times suddenly has a editorial line for this whole year that we shouldn't worry about voting fraud is also troublesome.
The nonreporting on the NewYorkPost story about voting fraud in Illinois is also a bit fishy.
That said, there doesn't seem to be concrete evidence of fraud in swing states that I have seen and the fact that Biden polled well ahead of Trump is another indicates that Biden is the likely winner.