Thanks to all of you for your responses and your willingness to share your practices (which I know have been developed through years of work and experience).

So here's what I gather from all of your posts:

1. Discuss/Educate your customer about possible unknowns regarding soil conditions

2. Consult an engineer if in any doubt about the bearing capability of the soil

3. Soils composition tests, while not 100% due to test locations, are helpful and a good risk management strategy for projects of any significant size/cost.

4. widening the footing is a good & cost effective risk management strategy