Assuming sublight, yes.
We will see an infrared spike from the launch site, and maybe approaching us. We will see another one when when they start breaking.
Assuming we ‘re looking, which ain’t that hard. We have the capability to do a “whole sky” scan every night in every hemisphere if we want to...
Depending on the aliens intentions, there may be a separation before they start braking. Hostile aliens will drop waves of bombardment vehicles before they start braking. that will pass through the solar system close to Earth and drop small projectiles to impact targets on Earth a substantial fraction of the speed light. A second wave will do bomb damage analysis and third wave will finish off what the first missed.
The good news is, if we pay attention, the velocity works both ways. At the speeds they are moving, we could probably destroy their bombardment vehicles with styrofoam cups if we could get them positioned in the right place (even a styrofoam cup is going to hurt at 30K KPS). Of course we could use more capable kill vehicles, too (blocks of iron are good...).
I'm ripping off Yer Pic;)
‘Enders Game’. We’ll just Shanghai some pubescent JDs from online war gaming rooms and train them to defeat the Formicans in one fell battle.
The rest of us can kick back with beer and BBQ ‘cause the little rodents won’t even know it’s all real.
They’re already here trimming my neighbors trees.