Firstly - I agree with Richard & ggcg's answers, that in order to sit in the zone, the only muscles tensed should be the ones you're actually using, not everything else, making you a tensed-up bundle of wasted energy.
Nothing good ever comes of being over-tense. If you're pushing too hard you will be screechy, or belty-in-a-bad-way, too far into your head, or shaky, or out of tune (probably sharp), your vibrato will be uncontrolled or unattainable, you will run out of breath. Not good for you & not good for your audience.
One thing that becomes apparent once you can get into that zone is how long you can hold a note or be able to run two lines together without running out of breath. This is a good judge of it, internally. If you're running out of breath, fading towards the end, or slipping out of tune, or you find you can't slide in & out of vibrato as you wish, can't vary vib speed or intensity, then slide back to no vib, then you're not relaxed enough.
Two fun exercises.
The big money notes at the end of Bill Withers' Lovely Day & Chris Isaak's Wicked Game. Neither is particularly difficult to sing, yet both have a good long note at the end, that you can tackle once you've used the rest of the song as a warm-up.
I wouldn't suggest ever trying either of them from cold. Use at least the full song itself to get you prepared & properly relaxed into it - then off you go. Each track gives you several shorter 'rehearsal takes' before arriving at the biggie at the end, letting you prepare for it in stages.
Neither is too difficult once you're in the zone.
Bonus points for 'playing' with your voice tone as you do it. Slide vibrato in & out, vary speed/intensity. Vary volume & intensity of the held note too, depending on your break-point move from chest to head or head to falsetto & back again.
I've actually been using the Chris Isaak as my warm-up song for about 25 years or so. I find it gives me a feel for how my voice is going to work for the rest of the day/night/session/show/whatever. It's gentle, doesn't need too much aggression & checks out whether you can open your range well enough without too much effort or any real extreme range. No belting, just control.
Added bonus for a live show on a strange stage, it lets you judge your mic proximity over several octaves & volumes for projection & on-stage monitoring. Very useful if you don't have in-ear monitoring & aren't on the monitor engineer's xmas card list for whatever reason ;)