Well, that depends on exactly what the term "rebuilt" means. Back in the old days, they would do what was called a "ring job". Some would say they "put rings and inserts in it". The "inserts", of course, meaning bearings. I've done a few myself. You use a ridge reamer to remove the portion of the cylinder wall above the ring wear line, then use a flex hone to scratch up the cylinders, and put it back together, with new rings and bearings. It's been done millions of times.

OR, "rebuild" can mean a cylinder overbore, new pistons, rings, bearings, cam, lifters, oil pump, etc, as well as a complete valve job, with new valves. It can also include the resizing of the cast rods, and using new ARP bolts. This resizing will shorten the rods slightly, which will reduce the CR slightly, unless enuff is milled off the block deck to make up for it. Some builders mill for zero deck height, or maybe .005, to allow for a slight cut on a future build.

So, all these factors, and more, can come into play when trying to determine the CR. For example: if the rods were resized and the 8-eyebrow or dish pistons were used, and the block was not decked, then you will have lower than stock CR. And the cam can help prevent, or help cause detonation. If the engine already has around 10:1 CR and you try to use one of the steep ramp cams, like the CC XE or Lunati Voodoo series cams, then the increase in cylinder pressure these cams cause, could easily cause detonation or make it worse.

These type cams produce more of what is called "dynamic compression" (DCR). Whereas a cam with more advertised duration, (one that begins to open the valves sooner, and closes 'em later) will "bleed off" some cylinder pressure at lower rpm, thus making detonation less likely, than with the steep ramp cam. These longer duration cams are said to produce less DCR.

For an example: in a 10:1 CR engine, a CC XE or Voodoo 262 cam would produce a much higher DCR than a RA4 grind cam, and therefore would be much more likely to cause detonation. That may be a crude explanation. But that's my understanding of how it works. So, you see, without knowing all the exact specs of your engine, nobody can say for sure exactly what the CR is or how likely it is to experience detonation.