Accretion disks play an important role in the formation of stars and planets and the chemical evolution of the protostellar system. The formation of RSDs is theoretically challenging and it is unclear from existing data at which stage in the star formation process young disks are rotationally supported. I will present new PdBI observations of 13CO and C18O toward two Stage I YSOs with resolution of 100 AU and much higher sensitivity than possible previously. A stable rotating disk is detected toward only one of the sources while the other is dominated by a pseudo-disk component. Two-dimensional gas and dust radiative transfer is used to model the physical structure of the two sources. The stable disk size is compared to two dimensional semi-analytical disk formation models. The same models are then used to search for possible observable signatures of RSDs in earlier stages of star formation. A number of diagnostics will be discussed which can be used to separate the embedded stable disk from the infalling rotating envelope.