We use the IRAM 30-m telescope to perform a sensitive search for CN N=2-1 in 42 T Tauri or Herbig Ae systems located mostly in the Taurus-Auriga region. $^{13}$CO J=2-1 is observed simultaneously to indicate the level of confusion with the surrounding molecular cloud. The bandpass also contains two transitions of ortho-H$_2$CO, one of SO and the C$^{17}$O J=2-1 line which provide complementary information on the nature of the emission. While $^{13}$CO is in general dominated by residual emission from the cloud, CN exhibits a high disk detection rate $> 50$% in our sample. We even report CN detection in stars for which interferometric searches failed to detect $^{12}$CO, presumably because of obscuration by a foreground, optically thick, cloud. Comparison between CN and o-H$_2$CO or SO line profiles and intensities divide the sample in two main categories. Sources with SO emission are bright and have strong H$_2$CO emission, leading in general to [H$_2$CO/CN]$ > 0.5$. Furthermore, their line profiles, combined with a priori information on the objects, suggest that the emission is coming from outflows or envelopes rather than from a circumstellar disk. On the other hand, most sources have [H$_2$CO/CN]$ < 0.3$, no SO emission, and some of them exhibit clear double-peaked profiles characteristics of rotating disks. In this second category, CN is likely tracing the proto-planetary disks. From the line flux and opacity derived from the hyperfine ratios, we constrain the outer radii of the disks, which range from 300 to 600 AU. The overall gas disk detection rate (including all molecular tracers) is $sim 68%$, and decreases for fainter continuum sources. This study shows that gas disks, like dust disks, are ubiquitous around young PMS stars in regions of isolated star formation, and that a large fraction of them have $R > 300$ AU.
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