Is it ethical to answer questions about homework for colleagues taking courses I've already completed? I receive many questions from colleagues about homework in a certain course which I took a while back. I try to answer them and help them solve homework which are very similar to the ones I did when I was taking the course. There is no TA for that course, so they come to me with their questions. 

I am not sure if what I am doing is ethical. The professor doesn't know that I am doing this. Should I stop and tell him about this and see what he says?
To "answer their questions" may mean different things, and some of them are OK, while others are not. E.g. answering questions like "What does this mean?" by explaining the thing in your own words is of course OK. However, solving their homework problems for them is not.

I guess, one can draw a line here: If you are helping them to learn the topic then it is OK. If you only try to help them to pass the course without caring about their progress in the topic, it is not OK.

So my advice would be: Focus on the topic itself when answering questions. Try to find out or guess what the instructor of the course wants the students to learn and help them to accomplish that.

Oh, and of course, asking the instructor is also OK.