From what I understand, SD cards 2GB and lower are 'regular sd'. SDHC is 4GB and above, although there are very few 'regular' 4GB SD cards, altohugh most of the 4GB SD card I've seen are SDHC.
Stick it in a device that only reads SD cards, not SDHC, obviously... SDHC cards are 4GB and larger, which narrows it down considerably. One would think the manufacturer could offer this information on the product's spec page...
If it's a 4GB SDHC card and you put it inside some older piece of hardware that can't recognize/read SDHC content, you'll know it. But, the twist: some older devices can't read over 1GB cards at all, and some can't read over 2GB cards... how's that for a wrench in the works?