Look at the Product Details on the book's page.
If it says "Simultaneous Device Usage: Unlimited", then it will be a DRM-free book. If it doesn't say anything, then it will have DRM. If it has a Simultaneous Device Usage but it gives a number of licenses, then it will have DRM.
Some examples:
1) Amazon's
freebie version of Pride & Prejudice which is in its Free Bestsellers list. Says SDU:Unlimited, so no DRM, and you can "purchase" and download/transfer it to check.
2) A freebie
Harlequin NASCAR romance that has been a promo for over a year now. No mention of any SDU, and it's DRM-ed if you download it.
3) A
"free" preview chapter that's a glorified sample that some publishers sneakily put up to boost the sales rank on the full book. SDU info says "Up to 5 simultaneous devices" and should you download this, it will be DRM-ed, unlike a regular sample.
Hope this helps.