You don't need to remember a ton of specifics before heading to the grocery store or farmers market. Some signs of freshness are unique to specific produce, but these will become second nature.

Use all of your senses. 
How does the item look, feel and smell? Look for a smooth exterior and vibrant colour; avoid produce with bruises and discolouration. Feel the produce carefully, taking care not to damage it. Over time, you can learn which items should be firm, and which should give slightly when pressed with a finger. Use your nose. If the nectarine doesn't smell sweet, it won't taste sweet (this is often more applicable to fruit). Sometimes you can taste before you buy, especially at a farmers market.

Avoid bagged, boxed and prepackaged items. 
It's hard to use your senses when there's a barrier between you and the produce. Also, rotting items that might not be visible can cause the other produce to spoil faster. "One bad apple will spoil the whole bunch," as they say.

Buy locally, in season
Seasonal produce will vary by region and be the freshest things available. Fruits and vegetables that have to travel across the country are often picked before they're ripe. Fresh equals flavour.

Ask questions. 
Don't be afraid to try before you buy at the farmers market. If the produce looks old at the grocery store, ask an employee to bring you better options from the stockroom.

The Washington POst