You can have LibreOffice automatically check spelling while you type and underline possible misspelt words with a red wavy line.
Choose Tools - Automatic Spell Checking.
Right-click a word with a red wavy underline, and then choose a suggested replacement word from the list, or from the AutoCorrect sub-menu.
If you choose a word from thesubmenu, the underlined word and the replacement word are automatically added to the AutoCorrect list for the current language. To view the AutoCorrect list, choose , and then click the tab.
You can also add the underlined word to your custom dictionary by choosing Add.
Select the words that you want to exclude.
Click the Language control on the Status bar to open a menu.
Choose "None (Do not check spelling)".