First, I would note that you don't need to know if it really is a drill or not. The admin team can perfectly prepare not knowing the exact time it will happen. That could be chosen exclusively by the person in charge to launch it.
Of course, you know that it is likely a drill. Most of the employees know that as well, even if actively told it is not. Coincidentally, there is a real emergency a few days after reminding everybody the evac rules. But, while you were preparing a drill for Thursday, a real emergency could happen on Monday!
So, I would probably acknowledge it with something like “It probably is, but just in case we still need to evacuate”.
Sometimes there is a warning in advance that "Tomorrow we will be doing alarm maintenance, please do not evacuate if they trigger." This is WRONG. What will you do if there is an emergency tomorrow? That would be the perfect time for an actively malicious action (e.g. a disgruntled employee), since your alarm system will be actively ignored. Your procedure needs to be robust and, if in doubt, fail safe.
You could for instance follow up after every alarm with a loudspeaker announcement: "please evacuate" or "this was a false alarm, please stay there". But, that should be done on every case. Don't evacuate if nobody tells you to? What if the guy that went to check if there was a real emergency (and those that went looking for him), fell unconscious due to e.g. Carbon monoxide in the room? This is a real threat that produced many casualties! Unless it is explicitly stated not to follow up, the default action (after giving a defined time for reaction) for the employees MUST be to treat it as something requiring evacuation of the affected zone.
A tricky drill scenario would be to trigger the alarm, say it was a false alarm. Then trigger a 'real' one in a few hours. Most people will assume (with no evidence) it is another misfiring, and "die" in the drill. Which is what would happen in the case where it is initially diagnosed as a malfunctioning sensor ("the smoke detector tripped, but we checked and saw no fire"), but the sensor was right, and there is a real threat which can end completely burning the building.
Some people have mentioned about people that need to know it is fake to "continue working". Well, that may be the case in very specific circumstances. That people should be briefed in advance and get confirmation after it is triggered that they are indeed expected not to follow the normal rules (a mass email to the affected people which is sent just after doing it would be a simple way to discreetly convey that). What are you doing which deserves such level of service? Are you in charge of fielding emergency telephone number calls? (911/112)
Even in that case, you should be able to evacuate the premises, having the service transfered to another dispatching office (with no advance notice!).
What can be done is to set expectations, like stating that everybody should be out of the building in 4 minutes, so they can politely finish the call with the customer before hanging the phone, pick their jacket and slowly proceed to evacuate.
As mentioned, people react somewhat differently when perceived as a drill vs a real emergency. seventyeightist mentioned in the comments people not wanting to break the
glass in a perceived drill. Some people could get outside, notice it's cold and go back to get their jacket "since it's just a drill". On the other hand, if they think it is not a drill, they could run over each other (despite knowing perfectly they should not run) or go take out their car to keep it from the fire (then causing the emergency services issues to reaching the building!).
Cort Ammon mentions that there could be a medical issue. If suspecting it is a drill, then it seems reasonable to get out-of-character and confirm that. Note however, that should also have been covered. Let's suppose that during evacuation someone breaks a leg. Option A may be not to move them. Option B to get them out of the building at any cost. Of course you will want different options depending whether it is a drill or not. But it also depends on the kind of emergency! The procedure should probably detail a way to contact with the emergency coordinating team, which would then direct the appropriate behavior: "Please wait there for the emergency services", "We are sending a couple of volunteers with a stretcher to your position", or even be asked to follow extraordinary advice "Firemen have indicated us that it is still safe to use the North elevator, so try to reach there and get out ASAP!". I dare to say this is also what should have been done in the server case.
You may have noticed that I mentioned many times how the procedure should have taken many things into account so that when the need arises, it is clear what needs to be done. I want to think that they are often good, well-thought documents, even though most probably when the day comes most people will not have read it (!) and will usually rely on learning how to act based on past drills actions and what is done by the people around them.
May thou have your evacuations be drills and not having to suffer real emergencies!