Matt is right about testing. As for cracks in the laterals, any crack big enough to let sand thru, you will see and it will be obvious. You mentioned that 'tape on the standpipe' trick so you must have been reading the archives. I think I mentioned that a couple years ago. For those that haven't seen the post, it is my suspicion that Hayward (my filter) designed the fit between the center pipe from the laterals and the top mount valve pipe to be a very loose fit. This lets air trapped in the top of the filter to bleed back to thr returns quickly. I added a few layers of black vinyl electrical tape to the top of the pipe to reduce the clearance but not seal it. It now takes a little longer for the returns to stop bubbling when I get air in the filter....from the vacuum hose when I connect it...but so what? I never had a sand blowby issue so I don't know if it did any good. But it didn't hurt things either.
Al