If we ignore the star child (seriously hes an abomination) if the reaper vanguard was not there aka sovereign, the only way for them to know would be any indoctrinated races that could communicate with them over that distance and/or them having a timer set to wake up and begin a harvest. The would be 0 way of them finding out with out sending someone into the galaxy to have a look around.
If we include the star child we are left with a mess. Why did the Star child aka the citadel not send the signal it self since "it" was in control of the reapers. Why even bother with Mass effect 1, the reason bad writing. We all know that though! Unless the star child figured that this cycle had the most potential for resolving the conflict of machine vs organics. So it decided to not interfere with the war and let the organics have a chance of finding a solution.
Either way I hate the star child.
*edit* thanks for the info regarding the codex forgot it listed some "facts" about ship speed size etc.