A tone above fs/2 reappears mirrored below it — 900 Hz sampled at 1 kHz reads as 100 Hz.
Frequencies reflect around the Nyquist frequency like a folded ruler. Anti-alias filters exist to remove them before the sampler ever sees them.
Deliberately, this becomes a feature rather than a fault: undersampling folds a 2.4 GHz carrier down to something a slow converter can read, which is how direct-RF sampling receivers work. The fold is only a bug when you did not intend it.
Estimate with the rule, then check it against the calculator that models it properly.
Open Sampling & Aliasing →