How do know whether the investments platform is authentic (other than those provided by the banks)?

The MAS regulation/licensing is a good thing, but there are also many useful platforms such as Conservation Capital which are not regulated. Of course, for safety reasons you can typically rely more on MAS regulated things.

If you're investing on other platforms, you may need to enlist legal help or demand specific forms that can protect your interests.

Maybe check whether they have a valid MAS license?