Don't ignore your nostrils.
I reckon the biggest clue will be the smell of burning.
I had a small piece of foam rubber insulating material become detached and fell onto the exhaust of a helicopter. The rubber started to smoulder and produce smoke and acrid fumes. These fumes came into the cabin whilst I was in the cruise and despite the fact that the exhaust was in fresh air behind the cabin. There were no other indications.
Needless to say, I stuck it on the ground sharpish! I was only at 500ft so I elected to keep the engine running although I auto'd to the ground and didn't need it.
I was always taught ( and indeed always teach my students) to confirm the fire is real and then shut the engine down, however, don't shut it down until you can make a reasonable engine-off landing site. That means the option of keeping it going for a few seconds to make it over the undulating ploughed field onto the flat newly cut stubble field for example.
Also the kind of smell is a big clue. You can't mistake electrical burning for anything else. Also, hot glass fibre composite has a trade-mark stench.
Finally, don't slam it into a field if you've just passed a power station or other such industrial polluter - sometimes the smells they give off can be quite disconcerting!
Happy flying!
JJ