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.
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.