Download h2testw,exe (it is free), plug the SD card into a USB port with a suitable adapter, and run the test program on it.
Depending on the memory size, it can take some time; but it will indicate if the card is real or a fake.
If the card is real, it may be pre-formatted using a file system the phone doesnt recognise. If you know what file system it wants (FAT32 for instance; you can download another free program called Aomei Partition Assistant, and use it to format the SD card with the correct file system.