Well in Firefox Nightly (v49) and the current Safari Technology Preview (5) CSS Grid is now enabled by default/unprefixed so you don't need to mess with any of that!
But for future reference you will likely encounter other things behind flags (prefixes are being killed thankfully but many things are still prefixed) so...
For features behind flags search in the URL/ominbox: For Chrome & Opera go to/search 'chrome://flags', for Firefox go to 'about:config', not sure about Webkit/Safari but they may not have any flagged features yet anyway. Then there will be a settings page with a bunch of features in development that you can turn on if you want to try them out, these can affect the stability of the browser so be warned.
For prefixes, just prefix the property or value with the vendor prefix (-ms-, -o-, -webkit-, -moz-) and use it as you normally would (there are occasionally syntax differences which you may need to look up). For example (and kinda redundant now):
body {
display: -webkit-grid;
-webkit-grid-columns: auto;
-webkit-grid-rows: auto;
}
There's a wealth of info for support for CSS and JS on caniuse.com, which i recommend checking out.