A good way to tell if you need protein is to do a strand test.

You take a strand of hair, wet, hold it at each end and stretch. If it snaps immediately or stretches very little, you need moisture. If it stretches 20%-30%, you probably have a good moisture/protein balance. If it stretches much more than about 30% and doesn't shorten up again (strand stays all limp and strectched out) you may need protein. I usually do a few hairs just to get an average.

Another way is that if you see your curls getting limp and dragged out (and you can't blame it on a low dewpoint) you may need a shot of protein.

If you are protein sensitve, your hair will feel like dead grass or straw when you use protein. In extremes, the hair can snap. It will feel dry and brittle, and adding more of a protein conditioner will just make it worse. Now, this won't mean that you are sensitive to all proteins. Wheat is a commonly seen as an offender, but some can use it. So, you may be unable to use wheat protein, but be OK with silk or soy or keratin.

The frequency of a protein treatment all depends on your hair. I had been using one once a week, but it's down to bi-weekly now. It all depends. That is something you'll have to learn on your own. The strand test can be helpful in figuring this out.

As far as ingredients to look for, I'd look for ingredients that you already like in your regular conditioner, but with protein higher up. Or, you can look for products marked as "reconstructors" (at least they are sold that way in the US) to find a high protein conditioner. If you are strict cg, do check other ingredients to see if it works for you.

As for something specific, I'll let the UK curlies chime in on that one.
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Med/Coarse, porous curly.