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So as the question states: do the Warhammer 40k factions possess a way of faster than light sensoring? More specifically: if spaceship A fires a laser at spaceship B which is lets say 10 lightseconds away, does B "see" that it is being fired at and might even make some kind of evasive maneuver?

And on a similar note on planetary defenses: as far as I understand it the distance between Terra and Mars for example is already approximatly 250 lightseconds, so around 8 minutes for a Laser. Therefore I assume the planetary defenses would have to seriously give lead to adjust for the targets movement if they want to fire at something approaching from the neighboring planet, let alone the edge of the system. Do they at least "see" the approaching ships with FTL speed?

Just a random thought I had when thinking a bit about some fluff story. Is there anything about it in the canon?

You can really dodge a lasgun bolt in the fluff? I mean... how? It travels at the speed of light, doesn't it?

Regarding FTL ships: I'm sorry, I think I failed to sufficiently explain what I meant:
Imagine a System similar to Sol: An enemy fleet drops out of warp somewhere around the distance of Neptune and begins approaching Earth. A sensor array with an astropath on Neptune warns Earths defenders and they switch on their defense satelites and space stations. At this point, with the enemy fleet approaching at sublight speed, those would need at least minutes, maybe hours to reach Earth. It would be interesting if the defense lasers in Earths orbit can already track them and try to fire some Lasershots (traveling at light speed) or if they have the same minutes to hours delay between the enemy fleets actual position and were they see them.

And viewing from the approaching fleet: the difference between FTL sensoring and not would be that with FTL sensor I can see the batteries on a planet I'm approaching firing before the lasers hit me (as the travel at light speed), without I would only see them firing the moment I get hit (as the flash from the shot travels as fast as the laser bolt/ray).

OK, that cleared up a lot of things somehow, thanks a lot.

The question that somehow still sticks is why would anyone in the HH approach Terra from the plane instead of "from above". If the defence spheres cannot really shoot outside of their immediate (in stelar relation) vicinity, why not skip Pluto, Neptune etc. and just drive directly to earths northpole.
Or on another note: since it is easy to calculate were a planet will be in a minute or an hour: drop out of warp, fire a huge load of lasershots at the expected position of the planet, go back into warp. They should only realize what's happening the moment the lasershots impact. But in the end, as you said: it does not have to make perfect sense physically.

Thanks for the comprehensive answer, that really covers what I was wondering about. I did not have anything specific in mind, I just could not really wrap my head around spacebattles in a setting with speed of light weaponry but without FTL sensors. But as you and others said: if engagement distances are short (astronomically) and lasers are not that massive, it makes a lot more sense.