If you have the following aspects in you're natal chart, it's likely you're left handed or have the capability of being left handed if you were taught to be right handed;
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1- Aspects from Saturn to Mercury i.e. any angle between these two planets. There are five main aspects(Conjunction, Opposition, Square, Trine, Sextile) and if you have them in your chart, then you will have some or all of the traits we have mentioned here.
2- Sign of Capricorn on either side of the 3rd house i.e. if your M.C. is Cancer or Leo (column D or E in your Moon's transits).
3- Saturn in the 3rd house
4- Mercury in the sign of Capricorn
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Out of the above I personally have;
1. Saturn(in Gemini) Square Mercury(in Pisces)
2. Capricorn in the 4th House
3. Saturn in Gemini(Solar 3rd House) but in the 9th natally.