It's easy enough to figure out if it was made with a brace by just examining the middle of the front and back sections of the tank's trim. I owned a 125G that was made without a centerbrace, so if the tank is VERY old, some were made that way. But if this is one-piece trim, it was most likely removed.
In a nutshell, newer tanks over 36" long are made with one-piece top trim that also includes a center brace. They were made that way for a reason. Removing the brace makes anything that happens structurally, entirely of your making.
"Why can't my wife see all of this stuff as an investment?"