  Forthcoming observational facilities will make the exploration of the early
universe routine, likely probing large populations of galaxies at very low
metallicities. It will therefore be important to have diagnostics that can
solidly identify and distinguish different classes of objects in such low
metallicity regimes. We use new photoionisation models to develop diagnostic
diagrams involving various nebular lines. We show that combinations of these
diagrams allow the identification and discrimination of the following classes
of objects in the early universe: PopIII and Direct Collapse Black Holes (DCBH)
in pristine environments, PopIII and DCBH embedded in slightly enriched ISM
(Z~10^{-5}-10^{-4}), (metal poor) PopII and AGN in enriched ISM. Diagnostics
involving rest-frame optical lines (that will be accessible by JWST) have a
better discriminatory power, but also rest-frame UV diagnostics can provide
very useful information. Interestingly, we find that metal lines such as [OIII]
5007A and CIV 1549A can remain relatively strong (about a factor of 0.1-1
relative H-beta and HeII 1640A, respectively), even in extremely metal poor
environments (Z~10^{-5}-10^{-4}), which could be embedding PopIII galaxies and
DCBH.
