Sometimes figures or clothing materials are not listed as iray or 3delight.
I have found mixing materials in a scene will sometimes slow down render speed.
The only way i know how to find out is to look for glossy layered weight, and that tells me its iray ( i guess )
How do you know what type of material your working with.
IF the asset is set up correctly, you can spot the difference via the "info" tag in the content library / smart content. The path to the material will show, and it will show either "3Delight" or "Iray" if the asset is properly set up. Note that this is also mostly a problem for older assets (older than G3) or non-DAZ originals (Renderosity and the like)
As a noob I have learnt that anything thats Gen 1 and Gen 2 is mostly going to be 3delight.
But If there's no RSL or MDL shown in the materials thumb nail pic then i don't know how else to find out what type it is. Some props, environments or hair will just list a folder with the name "materials" and nothing else.
I just thought there might be a way to identify the material type in the surfaces tab.