"Swing the elbow to tune the fifth."
Fifths are difficult to play on the violin since all fingers are at least slightly angled on the string, making moving the finger along the string (the traditional movement for changing pitch) ineffective.
Rock the elbow from left to right to find the optimal placement of the finger across the two strings.
As with octaves concentrate on the lower note and make it the dominant voice to help hear the 5th. To help this, make sure that enough of the finger pad is on the lower note.
When playing fifths also remember to keep a light touch with the finger since pressing the string down too hard can cause the finger to push the two strings further apart from each other. This will create even more of an intonation challenge!...