Questions to ask yourself when sorting out XRD spectra:
1) What crystal is it? BCC, FCC, HCP. Each type of crystal will have a series of forbidden reflections which can aid you in naming peaks in the spectra.
2) Do you know roughly what the crystallinity of the sample is? Is it one you have grown via MBE, for instance? Has it been grown via a bulk growth method?
3) Is your material a multilayer system? For instance a thin film on top of a tranisition metal or semiconductor like GaAs? If yes, you'll get peaks in the data from both sets of materials. This can help you identify the crystallographic orientation of the thin film relative to the substrate if it is an entirely new multilayer structure
It is almost impossible to be given a spectra and simply say "this is material [x] with [y] lattice spacing". You need to have some idea of what you've got so you have a starting point. Moreover, using a simple rocking curve (intensity vs 2-theta) is only of limited use. If you have a material you think is of decent quality with good crystallinity extended over large distances then you should take reciprocal space maps to try and quantify how crystalline the sample is and see if you have twinning or other phases in your sample.