The best way to way to spot these, if you're having trouble, is to hold the card directly under a lamp and move the card back and forth a bit. If you see anything that looks like a rainbow in the background, it's probably a rainbow foil. Regular base cards are just going to reflect the light source itself. Same goes with refractors, too. Specifically, the rainbow foil cards, will also have a metallic background that isn't present on a regular base card.
If you're just looking at a scan, it can be hard to tell the difference between a rainbow foil (or refractor) and its base counterparts. Depending on the scanner/how the card was scanned, it might just look "brighter" without the actual rainbow colors refracting back at you (this effect really requires movement and direct light source more than anything.)
