I have an H3 smart trainer. Normally I would say that comparable effort on the trainer has always felt about 10% harder than the same amount of watts outside (maybe due to erg mode). I took a week off the bike and now it seems to be dramatically harder - like 25% more effort required. It also seems to be louder than it was. How on earth can I possibly figure out if it’s a trainer issue, or just me getting an idea in my head?
Outside will naturally be easier for most folk you benefit from air cooling and inertia. I am one of the exemptions as I can switch off (not worrying about crashing) on longer intervals indoor and deliver more power. But the former is more prevalent 
Unless you have a power meter to test against, you don’t have much to do.
Make sure you have it calibrated properly is one thing.
Otherwise, maybe you are just feeling some extra RPE as a result of coming back from time off. And away from training pain.
Any other changes in bike, environmental or other considerations that may have lead to higher RPE?
Recently broke the belt on my H2, after replacing it the trainer is noticeably quieter and the same efforts feel slightly easier. Did calibrate after replacement but have not compared with on-bike power meter. My H2 is almost 3 years old and somewhere between 200 to 250 hours per year.
The first thing I did was recalibrate. I’m always skeptical of my own perceptions, but the trainer is undoubtedly louder than before. I just don’t know what to look for, of anything, to see if something is wearing internally.
With nothing more than that, you may need to contact Saris for support. Not sure how to troubleshoot more from here.