First of all, I ditched my red pen a long time ago. I read somewhere that red pen causes extra anxiety in students, but mostly I gave it up because I love the color green. Green is my signature color. So I do all of my grading in green felt tip pen.
When I am teaching my students something that I know they are going to have a hard time with, I tell them to do problem #whatever and then raise their hand to get a green star from me. This gives me a chance to check in with each student individually and see if they are on track. It also gives me a chance to help them correct any errors that they are making. It is simple, but it works amazingly well.
What is surprising to me about this little trick is that students really seem to care about getting a green star! Class will be over, and I will have a few stragglers lined up to get their green stars before they leave the room. On the occasion when I check someone's paper but forget the green star, they will certainly remind me. Sure, I have a few who don't understand and they don't want me to know that they don't know what they are doing. They will try to get by without checking in with me, but I know who they are and I make a point to check in with them.
My physics class is smaller and when I do this with them I usually have 3-5 problems that I want to check. So, they get a green star for the first one, a smiley face for the second one, etc. Then for the last problem I let them pick what they want. The requests have included a heart, a penguin, a flying squirrel, and beyond. Sometimes they tear off my little pictures and tuck them in their notebooks. So it is just a fun way for me to give each student some individual attention.