Some of the Cartomancy cards are a sign as well.
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The 2 or 3 of clubs (wands in tarot) shows someone shopping around.
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7 of spades is straight up the deception card, same as 7 of swords.
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Queen of clubs (Queen of wands) often shows up as the other person in relationship spreads.
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3 of hearts (3 of cups) suggests they’re out having fun without you but that doesn’t always mean cheating of course.
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9 of diamonds (9 of pentacles) is someone who likes being unattached, and Jack of hearts (page of cups) is what we call the young lover card.
Just remember that the Tarot is… complicated. Full of nuance and that’s where the real guidance from the cards comes from to begin with.
Plenty of cards could point to cheating depending on your question. If you specifically ask and pull the Jack of Diamonds (page of Pentacles), there’s an attraction forming. Jack of clubs (page of wands) means they’re already pursuing it. Queen of clubs suggests an ongoing thing.
The loyal cards would be stuff like the hermit, 4 of spades (4 of swords), or 7 of clubs (7 of wands). Those mean you’re good.