We know from the bones many things such as the weight the animal could support and how they walked indicating how they balanced. Depending on the age and sediment the animal might have even left a pretty good outline, and in highly alkaline conditions some saponified tissue would remain (completely denatured, but the shape would be there).

The creature in the OP meme for example was a hind-legged walker, so it couldn't be too front heavy and it's front paws weren't heavy weight-bearing. Just like all of it's close relatives who came before and after it.

HOWEVER! This interpretation can change! Originally only two Spinosaurus specimen were collected in very small number of bones with which to make recreations, due to difficulties in and around the region of Egypt. It can be very difficult for archeologists to make digs given their history with the region and the local extremist factions who oppose the concept of things like dinosaurs and ancient cultures. Recently a 2014 study which looked at one particular rock proposed the Spinosaurus tail was webbed, indicating it was semi-aquatic, which would have an impact on weight distribution of the animal and support the idea of a more bouyant build.