Paired t-test can be used only when the difference d is normally distributed. This can be checked using Shapiro-Wilk test.
This sentence come from http://www.sthda.com/english/wiki/paired-samples-t-test-in-r
In my daily work, I always use
t.test(data~group,alternative = 'two.sided',paired =TRUE) to paired sample t-test. I never do above mentioned pre test.
Does paired sample
t test need pre test?