no, you cant actually load it into photoshop and see whats been done. there IS no way to tell if something has been changed by photoshop. it doesnt leave any significant traces when saved...you just have to look for the obvious. usually pictures are very dull...thus, a rather vibrant picture will most likely be one thats modified...although not always. another thing to look for is the depth of field...people tend, in an attempt to create a focal point, to misuse the blur tool. in other words, is that boat half a mile in the background as in focus as the man two feet behind the focal point?
but no, there is no way to tell.