Look in Device Manager - under PCMCIA. If it says CardBus then it's 32 bit. Otherwise it's 16 bit. In general USB and CardBus came in at the same time so if you haven't a USB then you may well have a 16bit PCMCIA slot.
32bit cards won't fit a 16bit slot (16bit will fit 32bit slot) because of some bumps near the connector end of the PCMCIA card.
I use a 16bit card with an ancient Toshiba Satellite 4030CDS.
If in doubt buy 16bit - it's just as quick over 802.11b.