My AH has been addicted since the age of 13 and heroin joined our lives about 4 years ago, following an addiction to prescription drugs. First, I am sorry that you are going through this. You will continue to be in my prayers. Second, you will not need to look for signs - listen to your gut. It is hard at first because you don't want to believe it, but the reality is that if you think he's using, then he probably is.
Some clear heroin signs, pupils are pinpoints regardless of the amount of light. Stares off into space - you can be talking to the addict and feel like they are looking directly thru you. Coming off heroin causes leg cramps, nausea, sleeping, sweating, etc
Please remember that this horrible habit has NOTHING to do with you, and his lies aren't a reflection of you, they are what an addict does to achieve their high. Nothing is more important. Please take care of yourself.