Make sure the area is loose and you have ROM before oyu worry about strength there.
Indeed. Good condition of the hip flexors trumps strength. I spent years in college doing hip flexor strengthening exercises to improve the ease of my stepover. It barely helped at all. Years later when I learned how to stretch that area (see Stretch To Win book) and/or get therapy there, my stepover and ease of hip flexion significantly improved very quickly.
By exercises I am referring to dynamic stretching or mobility exercises. Are there any ways to test how your flexibility mobility is in that session? Or in absolute terms where it should be?
Sufficient joint mobility will manifest itself via improved mechanics at speed as the dynamics at speed are a world apart from what happens in more of a passive environment.
Once you’ve reached the stage in which the athlete is not demonstrating signs of being impaired by mobility issues the assessment as to whether it’s up to par for each respective session will entail some trial and error; which is why it’s imperative that the coach/trainer/athlete have the where with all to make the necessary adjustments on the fly.
I’ll typically have my athletes perform a uniform warm up and finish with giving them time to do what they feel they need to do on an individual basis. From there, if there’s any lingering issues, I’ll typically perform some PNF moves on them and that usually does the trick.