If the first character, the start sentinel, is a ';' on 5-bit/character tracks, and a '%' on 7-bit/character tracks. To be a valid track, the track must have a valid start sentinel, end sentinel, and longitudinal redundancy check character; and the parity on each character must be valid. Any track with 16 or fewer bits of data is invalid, the data are treated as noise.
So if you card has a single track of data beginning with a ; followed by a number (multiple numeric digits) and terminates with a ? It WILL be seen as a financial card. This is not the only card format that will be seen as a financial card but it is the most common.
You can find more information here:
and here is the ID TECH document that speaks to the issue on page 20