back on topic:
you find the check for problems feature under MAP/check for problems(up by file, edit, view, etc...)
or you use the hotkey ALT+P
btw...it is not only for manipulations. i cant believe you made 4 maps and dont know about this yet
glad you do now tho...
you should do a check for problems EVERYTIME once before you feel like exporting to .map. many problems may appear here other than invalid solid structures. most of them can be "fixed" from here with the fix button.
most common are the texture "axis perpendicular to face", just highlight it and "fix"(happens after clipping carving hollowing vertex manipulation) you also get another texture mistake by doing any of the above that is also common, invalid texture <texturename>. this is the error that appears when one of your faces turns white after carving, clipping, hollowing and vertex. this can be "fixed" too. best way to fix this is finding the texture in the texture application tool from your wad........selecting it, then checking for problems and fixing it with this texture. if you had another texture in the texture application tool....you would give the "fixed" face that txture...not always the desired result
some entitiy problems will show up here too(unused key values) and (no players start). the latter isnt really a problem so just add a CT spawn. i think very few will understand unused key values....ive never seen where they may be in my entities but itll happen none the less. and also will notice bad target fields in entities. so thats a good one.
peace