Your plants will probably tell you if they need magnesium, typically they will develop interveinal chlorosis, yellowing leaves with green veins, usually on the oldest leaves. If it's on the youngest leaves it is more likely to be iron or manganese deficiency. There are some pictures here:
https://www.google.ca/search?q...
Are you seeing something that suggests a problem? The cost of soil testing varies widely, some labs charge a lot as Elaine said, others are much less.
