The NEC says to follow the glue manufacturers instructions. Here is a thread from the Mike Holt electrical forum on gluing:
http://forums.mikeholt.com/showthread.php?t=135940 I don't see anything where the code specifies that the glue must be visible. I'm an EE (Canadian) and have never heard of that here. Once you have all the conduit and fittings connected and attached with straps, etc.(above ground) it's not likely to come apart and gluing the sn*t out of a joint won't help much.
Glued joints, whether or not you can see the glue after assembly, is no guarantee that the joint won't leak anyway, esp. underground. I've seen some old joints where you can relatively easily pull the pieces apart. For a better seal, some will use PVC primer like they do in pressurized plumbing systems. Primer will melt the plastic providing a good bond between surfaces. I've never seen an elctrician use primer.
If still in doubt, the Mike Holt forum is an excellent place to ask.