After having formatted the cards to exFAT, I test microSD cards the same way I test other storage components (external HDD, portable SSD etc). I use the latest versions of CrystalDiskMark, Atto, AS SSD and AJA benchmarks, noting the best scores achieved in each. They are all free and can be downloaded by anyone. MicroSD cards are usually divided into classes, giving a rough idea of the minimum performance to be expected.
A Class 4 card is guaranteed to deliver at least 4MBps, while a Class 10 should surpass 10MBps. Similarly, many vendors report their speeds in terms of "x" rating, which is a multiple of 150KBps (the read speed of a standard CDROM drive). A 100x card can be expected to deliver more than 14MBps.
After these so-called synthetic tests, I then transfer a bunch of files and folders, totaling around 10GB, to get a real-life transfer speed using the latest version of Fastcopy.