Write pseudorandom data to the whole card with a fixed seed.

Read back data and compare against the same pseudorandom data with a same fixed seed.

Or just make a random data file the size of SD card and write/read back compare, if you don't care about having to store the big file and also testing for potential local disk corruption at the same time.

Or use https://github.com/dkrahmer/MediaTester

Looks interesting but unfortunately a windows tool. Happen to know a Linux tool?

I'd rather use something that works with block device directly, rather than something that depends on the filesystem code and may lead to filesystem corruption and potential for kernel instability. Also it seems like a weird design decision to fill flash with files, when in Linux there's trivial access to block device directly.

It's also possible to write 64bit address of each 8 byte block to every such block, and avoid pseudorandom generator, and potentially have more insight to what happened when the block ends up mapped to unexpected location.

The first thing I do is a simple format and check the formatted capacity.

Cheap, fake cards will often fail this simple basic test.

There are a number of readily available utilities to further test performance and reliability.