Matching piece locations are only part of the test. The side to move must match, and the legal options must be the same. Lost castling rights or a legally available en-passant capture can distinguish two positions that look identical.
The initial position can count as the first occurrence. A complete game score lets you reconstruct later positions; one screenshot or FEN cannot establish how many times a position appeared. Repeated moves or three checks do not by themselves prove repetition.
Under FIDE competition rules, threefold repetition requires a correct claim, including the prescribed procedure when an intended move would create the third occurrence. Fivefold repetition is automatic. Consult the arbiter in an over-the-board event; an online platform may automate claims differently.