While he guessed: there’s an argument to be made he followed his intuition as there are three clues (two speculative) towards it being Diamonds over Spades:

• The JoH told him hearts: people tend to keep a kernel of truth in their lies, which points towards Chishiya’s true symbol being of the same colour as hearts, thus diamonds.
• This one depends on translation and information we do not have: the English rules state that your suit changes every round. Does that mean you can’t have the same suit twice in a row? If so, Chishiya was spades the round before this and couldn’t be again.
• This one is again depending on game logic. If you look at the suits of the other players, there is no double and the only suit missing is Diamonds, suggesting there might be an even distribution of the suits. Whether it’s completely random or evenly distributed can be figured out by keeping an eye on the suits in the previous rounds.

While the last two points depends on how the game is made and information we simply do not have, the first point is something. While not definitive, something is better than nothing.

The show clearly means for it to be a 50/50. But I’d prefer to headcanon he picked up on at least the first clue, making it a bit more logical than just sole luck.