Well, you probably wouldn’t want to see the copious output of a print function during rendering. Is that the context you are talking about, or is it something else?
There is a little-known utility called debugcal, which you can use with a rendered image to get some level of debugging like so:
ximage -e auto rendering.hdr | debugcal -f myfile.cal -e 'arg(n)=select(n,A1,A2,A3,A4)' -e '$1=testvar1;$2=testvar2'
You have to assign the A1,A2, etc. arguments to match those in your pattern or whatever you are debugging, and the ‘$1=…’ arguments to match the variables you want to examine. You can then click the middle mouse button or hit the ‘t’ key in ximage over the pixel(s) you want to test. It’s a bit of a manual process, and only works on Unix boxes, but it does work.