how can I tell the difference 1998 LS or GS?
Nothing definitive, each trim level you can get different options....https://en.wikipedia.org/wiki/Mercury_Sable#Third_generat ion_.281996.E2.80.931999.29
You can call ford customer service, they could tell you, have your VIN handy.
Search Mercury Sable Questions
Mercury Sable Experts
Related Models For Sale
Used Cars For Sale