The "forgiveness rating" of a game, or its "cruelty," will tell you whether the game can be made unwinnable. (You can read more about the cruelty scale on IFWiki: http://www.ifwiki.org/index.php/Cruelty_scale)
To find games that can't be made unwinnable, look for games with a rating of "merciful." You can search the Interactive Fiction Database for merciful games by adding forgiveness:merciful to the end of your search terms: http://ifdb.tads.org/search?searchfor=forgiveness%3Amerciful