Question

If you are able to see an object, how can you be sure that it is not at absolute zero?

Answer 1

You can't ever be 100% about anything, but you can assume with a 99% certainty that it isn't because getting to absolute 0 would be like getting to the speed of light. It's an unattainable constant, you would have to constantly subtract an infinite amount of heat which isn't physically possible.

Answer 2

plus you can always just measure the temperature of the object