While reviewing it shows you if a card is a leech by looking at the review history
While reviewing it shows you if a card is a leech by looking at the review history and if it was (in the default config) failed 4 times you get a big reed leech indicator. (I only count one fail per day)
You also have a little counter under the pitch accent showing you how far it is from becoming a leech. I named it Contamination.
It also only starts counting after (in the default config) 4 correct answers
Those cards get a green new indicator (can be disabled in the config)