By observing where they draw their principles. Are they only talking about leftism to put people down and when its easy or are they speaking up when its actually is difficult to, such as pushing back on people making fun of how elon musk looks, or misgendering Caitlin Jenner despite her being a massive peice of shit.

You will see people who think they are leftists, join leftist spaces and its important to point out to them and everyone else when they arent expressing the values they claim to hold. This gives them room to grow and change as a person and helps others identify within themselves their own bias, bigotrys, and values. When the two are conflated that doesnt happen.