I'd figure it out by the "soft white" dusting inside the glass. Obviously it would be easier with them turned off but I could figure it out, on, as well. LEDs have a more plastic-y diffuser.
The lower watt incandescent bulbs are way more distinguishable. I had some leftover 40W and they have a very bright “core”. The higher wattage ones are very hard to tell, if you don’t know what you’re looking for. In other words, if I was in someone else’s house who was using these filament-type LEDs, I’d probably think to myself: ’this guy is still using incandescents everywhere!?’
Seriously, if you haven’t seen filament-type LEDs, you have to in order to understand. There is no weird reflector, fat base or anything to clue you into the fact that they’re LED if you haven’t seen them before. Even if you have, it would be difficult to the average person.
Part of my point, is that a lot of people refused to (or begrudgingly) adopted CFL and early LED tech, because they liked how incandescent bulbs looked. If you’ve got the money and we’re willing to pay a small amount more for electricity, there was no real benefit. THESE LED bulbs are so close that the arguments against them rings hollow.
Now, some of the LED bulbs I’ve purchased have failed way too soon. HOWEVER, altogether, the ROI is quite short. Except for infrequently used bulbs, there’s little excuse to NOT literally throw away old incandescent bulbs in most cases; the exception being like the incandescent bulbs I’m using in ceiling fans, with the fan on, and in a house with all-electric heat anyway.
For bulbs that stay on most of the day the ROI is well less than the warranty period, so there is zero reason to be a hold out.
Finally, I can totally understand wanting to maximize the use of mercury-containing CFL bulbs for environmental reasons. For me, the bulbs will eventually end up being properly disposed of anyway, so I selfishly choose to save money now by using LED bulbs.