How does plot.lm() determine outliers for residual vs fitted plot?

They locate the largest 3 absolute standardised residuals. Consider this example:

fit <- lm(dist ~ speed, cars)
plot(fit, which = 1)

r <- rstandard(fit)  ## get standardised residuals
order(abs(r), decreasing = TRUE)[1:3]
# [1] 49 23 35