It depends on what you are growing, but typically a tomato plant hit by frost will get droopy within 24 hours. Sometimes it's just the most exposed outer leaves of the plant but the time/duration and lowest temp factors into the outcome.

Peppers tomatoes cucumbers squash lettuce potatoes

Your lettuce should be ok depending on the low temp, 20's may hurt it a bit but wipe out all the others you listed.

So far sunflowers have done great. One I poured warm water around to help warm it and area. That water froze hard on a leaf - still ok. Slight dark patch. I always read sunflowers are not frost tolerant but they took it well. So far things looks ok. I know when my rosemary freezes to hard it takes weeks to be obvious it is not making.