There are quite a few things really, I'll list three here, they are specific. I even chose three that aren't him advocating for government run grocery stores...
Let's begin with the speech of his during the 2023 DSA Convention where he emphasized "seizing the means of production" as an end goal, a phrase directly tied to Marxist ideology. Then there is the reality that the first use of "Socialist Democracy" was in the Law of the Soviet State (1938), it specifically refers to the U.S.S.R. as a Socialist Democracy 40 total times. The Bolsheviks were literally members of the "Social Democratic Workers' Party".
In a 2020 social media post, he praised a communist mayor in India, suggesting New York City needs a similar leader.
A 2021 video for the Gravel Institute shows him advocating for the "de-commodification of housing," which some interpret as a call to abolish private property, another core communist principle.
There are more signs, but the guy is openly further into the communist manifesto than Bernie Sanders (I don't believe he is actually further left than Bernie, just that Bernie will not be this open about his plans).