(one's) little finger told (one) that British spoken language informal
The speaker uses this phrase to admit that he or she has learned something by an unconventional way.
I knew that it was going to rain, my little finger told me that.
This contract looks good on paper, but my little finger told me that it is not.
Those people look suspicious, my little finger told me that.
Used to indicate that it is very easy to guess something
Relying on guessing and luck
In the past, an uncomfortable feeling in the little finger is usually associated with things which are going to happen.
To do something in an unusual way
I guess I like to run against the grain in everything I do.