There are a few ways to turn on spell check on your Samsung.
First, you can go into the settings menu and look for the language and input option. Once you click on that, you should see an option for spell check. You can turn it on from there.
Another way to turn on spell check is to go into the keyboard settings. On most Samsung keyboards, there is an option to enable spell check.
If you're using a third-party keyboard, the process may be slightly different. However, most keyboards have some sort of spell check feature that you can enable.