There are several types of irritation studies:
1 Patch testing. They place several dilutions under adhesive tape on the skin. Leave for 6-24 hours and see which ones caused allergic reactions from redness to blisters.
They may give a false positive for rinseoff products, which won’t cause much irritation if quickly and completely rinsed off.
It ain’t that expensive or complicated, but at least hire a pro to apply it and check the results.
2 Normal use testing. They ask volunteers to use the product as intended. They then check for redness or irritation.
Not too expensive or complicated but usually requires more paid volunteers than #1.
3 Cell culture testing. Expensive, complicated and may be unrelated to real world use.
You can provide them some info from the separate ingredients, and extrapolate to the finished product.
If the client pays for it, you can arrange some patch testing. Just warn them that even incomplete rinsing ain’t as bad as reactions behind an occluded patch.