How Do You Know When the Load is Too Heavy?

For a front end loader or fork lift, how do you know if you are trying to lift something that is too heavy for your equipment?

Do you:
1. Know the limits of the equipment, estimate the weight of the load and don't try to lift something that is at or exceeds the limit.
2. Just try to lift the load and if it works you are ok.
3. Observe how the tractor is groaning, popping or bogging down to determine that the load is too heavy.

If your answer is #3, what do you listen for?