The 32-year-old model was recently spotted wearing a mystery ring on her engagement finger.
However, Brook is known to have worn innocent bands on the same finger before.
The Piranha 3D actress has been dating Evans since November 2010.
The couple were hit by tragedy last year when Brook lost their unborn baby more than five months into her pregnancy.
Brook has since said that the horrific experience did some good in that it brought the pair closer together.
"All of a sudden you find yourself with someone you don't know that well, and you're in that situation," she said. "It was a beautiful thing, but a really scary thing, and I think we probably saw each other at our worst... I owe everything to him, really. I'd do anything for him."