If you want to see if it is your FTP, go try something like this:
IMO, if your FTP is correct, a workout like this should be manageable, solid HARD, but not soul crushing Very Hard / All Out.
(Try it at your existing FTP first, at worst, it’s a mild Sweet Spot Workout…)
I did manage to average 284W for 47 minutes up a climb today (and then do another hour after it). By no means a full-bore effort, but certainly “hard”. I think 300ish feels right 
Resurrected thread, but I’ll add my typical input:
Just do an FTP test.
Youre so old school
Doing ftp test is too scary. Much better to guess and hope you get it right.
You make it sound simple, but which test…? So many definitions and protocols to choose from.
I actually find TR AIFTP surprisingly close to my perceived FTP. Intervals.icu requires a max effort and typically sits significantly lower since my training/racing seldom includes an “all out” interval.
With some over/under work and high sweet spot training, I typically know where my threshold sits within ~5-8 watts. I’ve done lab testing a couple times with blood lactate monitoring and I’ve been really close to my “perceived” number. With experience and reasonably fresh legs, it’s pretty easy to dial it in and feel the cross-over point as you exceed threshold and start digging a hole. I also subscribe to treating threshold as a physiological state (MLSS), not the squishy 40-70 minute or “about an hour” definitions.