If you have some sort of decent antivirus/antimalware like Malwarebytes, that would work for standalone applications to an extent.
Browser extensions are a lot harder to check.
Always make sure you get the RIGHT extension from the PROPER SOURCE. Same with most downloads and app store stuff on your mobile devices, but at least with executables, you can additionally run virus scans for some peace of mind.
Some tips…
Always make sure you’re accessing the extension’s download/install page from a trusted source.
Check reviews AND READ THEM. Make sure they don’t look suspicious/bot-generated.
Consider what permissions you’re giving the extension. Your browser has a lot of personal and sensitive information… including the “keys” to a lot of your accounts. Basically any website that you don’t need to sign into every time you access it? That “key” is stored in your browser.
I believe you can generally trust what permissions an extension or app needs (since the browser/device knows which permission an extension/app uses, and locks them away otherwise), but be wary of the implications of some of them (such as data from other websites, or accessibility features).