Possible Clues:

• Adjective modifiers
• Always and forever
• Perhaps 'hurry, compare and buy' and 'fairly and squarely' for example
• Sometimes and often
• Many end with '-ly'
• Tom Swifty words
• They qualify subtly and accurately, maybe
• Most words ending in '-ly'
• Say, rather badly: 'Bread vs rocks'
• Now and then, grammatically