They can tell whether you fired A gun, not necessarily that you fired that specific gun into your specific body. It's circumstantial evidence at best.
Better evidence would be signs of a struggle or calculating bullet velocity to see if it came from the gun in that position. However, there are ways of doing this without actually causing a struggle. Wait for them to go to sleep; you could even give them some melatonin or something to make them drowsy. If it's in a car, put some nitrogen canisters on the A/C intake to make them pass out.
If it is nefarious, there will be evidence somewhere. Whether you care to look for that evidence, know where to look for it or have the technology to find it ... that's another matter altogether.