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System Profiler > Hardware > Serial-ATA and look up the model number for the SSD.
In the following picture it is a Hitachi drive, so you search for "HTS...." or whatever is listed there for you.
mac-os-x-system-profiler-2-sata.png


Also most SSDs are 64GB or 128GB or 256GB or 512GB in size, HDDs are more like 120GB, 160GB, 250GB, 320GB or 500GB in size.
 
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