If I get married, would any stocks or investments I have before marriage become community property? Could I risk losing them in a divorce?

Typically speaking, property you bring into a marriage can remain separate property, even in a community property state. However, there are usually "rules" that you must follow to ensure that it does not become considered community property, and there are also rules that dictate how separate and community property will be treated in a divorce.

Therefore, you really need to speak with an estate planning/family lawyer in your state to be certain of the exact rules and exact treatment of the property in question. Laws can vary from state-to-state (e.g. the community property laws in California could vary some from the community property laws in Texas) - there may not be much difference, but any differences could potentially have a major impact on how property is categorized and how it is treated before, during, and after a marriage.

Your assets become community property based on the state where you live. California, for example, is a community proeprty state. If you are married in California and move to Massachusetts (a non community property commonwealth), your assets become joint assets.

There is little short of a pre-nuptual agreement that will guarantee against loss in the event of a divorce no matter where you live, and I have seen pre-nuptual agreements invalidated or thrown out.