We've worked with SWAT, and one of their commanders offered the following about real vs. fake cops crashing your front door. This is something that has happened in Arizona lately. (either way, real cop or not, you have to decide what are the consequences if you 'chose wrong' with how you're going to respond)
In many jurisdictions, warrants can't be served late at night, which can be a big hint if they show up at 3 AM.
Real cops should have matching gear that looks high quality (vs. a $5 POLICE T-shirt) and a cheesy .25 cal saturday night special.
He noted he was not aware of any instances where real US cops carried AK-47's, a weapon some fake cops have used.
The SWAT guys will be virtually identically dressed with Kevlar helmets and $5K Motorola radios. Real cops serving a warrant will likely have sporadic traffic on their radio.
Additionally, real cops typically arrive in real cop cars, either themselves, or just in front of a traffic cop in a cruiser to dissuade onlookers from getting too close.
For non-high risk warrants that don't require the full SWAT team, you might have 2 or 3 SWAT guys serving it in something short of full battle gear.
Warrant servers have on occasion allowed the subject, often barricaded, to call 911 to verify they were real, generally in drug / kidnapping situations.
The challenge / potential downside is that unless you're really sure, I'd be very careful trying to kinetically challenge the intruders on very short notice. I would expect most TFL'ers could shoot better than the average street cop and certainly the average thug, but I expect few could out shoot the SWAT guys in a tactical situation of their choosing. May each of you never have to find out.