Sorry it took so long to get back to you. My computer blew up a few days ago, and it’s been a long haul getting back up and running. As to your questions; I’m not an expert on concrete – just an owner-builder – so take all these comments with that in mind.
Is there a way to test before pouring? The concrete gains its strength as it cures. I don’t know of any common ways that residential concrete is tested prior to placing it. Heck, it is very atypical to test it at all! We tested the strength of the slab in our farm shop for several reasons. One, we told the contractor in advance that we would test it, so he knew that there would a quality-control check. Concrete gains strength as it cures and we planned to slow cure the slab, so having several samples to test would also inform us as to when the slab had reached full strength (but in the end we tested only one sample). And finally, my husband is generally curious about such things.
That all being said, though, there are some things other than testing that you could consider. First, decide what strength concrete (psi) you need or want, and include that info in your request for bids (this may be a code issue). Second, when the concrete truck shows up on site, ask to see the paperwork from the plant to make sure that the load in the truck matches what was ordered. Third, observe work in progress. For example, if they want to add water to the load on site to make it more flowable, that could mean reduced quality and strength in your final product. Make sure that someone on site, whether it is you or the contractor you’ve hired, will be on top of the quality issue and be willing to refuse delivery if needed (e.g. it sat in the truck too long and started to set). There are a lot of details and logistics involved in concrete work, so consider hiring the best contractor you can find. But even with that, being familiar with the process and terminology and making sure you’re on site the day of the pour are all part of the equation.
What if it doesn't meet specs? John told me (after he read your questions) that had he fully intended to have the contractor jackhammer out the slab and replace if it didn’t meet specs… but I think that may be a bit of male bravado talking there. ;-) I’m sure no one, not you and certainly not the contractor, wants that type of disaster on their hands. I know there are horror stories out there, but overall I think that thing is pretty rare.
Is it recommended to test footers and foundation walls, and how is it done? I’ve never heard of testing residential work in our local area, but I have very limited experience. In our case, John took the unlabeled sample (so they didn’t know what job or what strength it should have been) back to the concrete plant. Apparently they pull samples themselves from their own batches to test for quality control. They tested the sample for free right in front of him. If we had wanted more samples done, I think there was some nominal charge. John thinks they did it for free because it was just so darn weird to have someone walk in off the street and ask for a test. Overall it was easy to do, interesting, and may have kept people more on their toes that might have happened otherwise. But keep in mind that in addition to what comes in the truck, how it cures plays a big factor in the final strength. In our case, we could wait on any further use of the building while we watered and slow-cured the slab – but that is not likely to be the case in a footing or foundation.Good luck on your build!