You can have LibreOffice automatically check spelling while you type and underline possible misspelt words with a red wavy line.
To check spelling automatically while you type:
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 submenu.
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.
To exclude words from the spellcheck:
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)".