How to diagnose the SMC? This article from the Apple Knowledge Base:
Reset the System Management Controller (SMC) on your Mac

describes very well which kinds of events are symptoms which might
be related to an SMC in need of a reset. But the relationship isn't
clearly established.

Is there any simple method to diagnose clearly the SMC state:


is it OK?
does it need a reset?


Something like (for NVRAM):

nvram -p


or

nvram -p | diff /etc/nvram.backup -
Your best bet is to go through Apple's diagnotic utilities, but be prepared for it to tell you everything is fine. 

Put you machine on a flat stable surface and plug it in.  Perform a restart or a power on and immediately hold down the 'd' key (d for diagnostics).  It should at least test your power adapter which is goverened by the SMC chip.