they stop eating a while before they molt.other than that im not sure of any other indicators. actually, i just remeber a while back reading something about how the pede outgrows its shell and this causes it to look like it has a "neck" before an impending molt... im not sure if this is true, maybe someone with more experience can verify this?
oh i jus trealized, the thing i was talking about with it growing a neck or wahtever. thats on the centipede general info part of this site. you should check it out. it has alot about molting on there.