Irreversible pulpitis is largely diagnosed by signs and symptoms.
There are often visible radiographic in the pulp spaces.

Symptoms include thermal sensitivity, esp lingering sensitivity
to cold and sensitivity to hot. Sensitivity to percussion is also
usually present.  Tend to go by the severity of the symptoms
and also check other factors that could be causing sensitivity,
including checking the occlusion <bite> to see if it could be
traumatic occlusion (can also be a contributing factor)

There are just too many factors that go into an evaluation
to go into here but the above may give you some hints.

If you could post an x-ray would be glad to evaluate it for you.

Getting sent for an endo consult gives you a pretty good idea
that you may need RCT in the future.

You can try this test yourself:  place a small piece of ice directly
on the questionable tooth and not touching any other teeth.
If you have a 'violent' reaction to cold, then you should
start planning for RCT.