The WebSDR suggestion is brilliant
(Never even occurred to me and great idea if one is in range.)
Unless you can hear it for yourself or from an actual trusted source you'll be lost.
'Scratchy' is hardly a very descriptive word for audio quality unless you're playing an old beat up vinyl record
The operators that said your FM was off, was this on UK CB frequencies?
If so there is always the old '125' argument and usually the oldest of human shortcomings, namely 'elitism'. I mean, why would your more modern 'Ham' rig with decent VFO and frequency display be any better than their old beat up 1980's CB?
I hear the old "You're off frequency" line a heck of a lot. Used to be constrained to CB but you hear a lot of it on the Ham bands nowadays, (people sticking to the 0 & 5 channelised mentality).
Don't fret about it until you can truly confirm it for yourself, that "seed of doubt" that's been placed is a terrible thing to do to someone unless they are genuinely trying to help.
An old 'trick' I've used in the past when I received a report of poor audio was to ask them to wait whilst I change microphones. Then I'd pause transmission, key back up and ask them how the 'different' mic was, (whilst never actually changing the mic)....you'd be amazed how many said it was now better!!
With that said, try not to be too quick to judge or call people out over it. It happens
Hopefully your rig is fine and you've just experienced poor reporting.
How many 'Hams' give 5/9 reports all
of the time yet ask you to repeat your last over?
Again, don't fret unless you can truly confirm it for yourself.