Usually when guessing someone elses sign one might not really guess or tune into their Sun Sign at all but in fact the sign where their Atmarkarka-planet is placed.
The Atmarkarka is a concept in Vedic astrology where the seven embodied planets become “re-categorized” for each horoscope – organized after the degrees they have. The planet with the highest degree in the Birth chart become like the Sun for the person (Number 1).
The reason for guessing ” wrong” Sun sign is simple – it is usually not(but can be) the Sun or a planet with the Sun (in the same sign as the sun) that is their soul planet or personal “sun” planet, thus the sign where their actual personal ” sun” planet is placed is always more prominent in a the persons chart and in their expression.
Personally I have had that happen a lot of times – the sign that I thought a person was (sun sign) was in fact totally wrong, but my intuition was right – because usually that sign that I felt they “were” was usually the sign where they had their Atmarkarka instead. Only if the Atmarkarka was the Sun or placed in the same sign as the sun one might appear and be as ones sun sign and thus guess “ right” sun sign.
Something else I also have discovered is that when I get like: “oh, shit this celebrity looks exactly/reminds me exactly of this other one” – when I compared the two I usually saw they had their Ascendants and/or their Atmarkarka in the same Nakshatrastar.
And so I have become more and more aware of that it is your Atmarkarka sign(and nakshatra) and Ascendant sign/nakshatra that is always the most prominent in a person.
In Vedic and Western astrology there is a lot of focus on Sun and Moon sign – but as I have studied charts they are not the most visible or prominent outwardly in their character – rather their Ascendant nakshatra and Atmarkarka nakshatra are very visible for others to see.