FAX programs can use the higher speed, but only for sending FAXes. This can give you a clue about speed. If your modem supports the V.32 protocol, it is a 9600 BPS modem. If it supports V.32bis, it is a 14,400 BPS modem. If it supports V.FC or V.34, it is a 28,800 BPS modem.
Faster modems are usually backward-compatible with all the slower speeds.
If your modem documentation lists many protocols that your modem supports, you can determine your modem's highest speed from your modem's highest protocol. See also DTE Speed.