I cant trust my husband how can I find out if he's faithful when he's at work?

Hire a private detective.

Really, this is a strange question. What kind of marriage exists without trust? Why are you married to him? What proof do you have that you can't trust him?
Do you keep busy, do you have a job, or kids, or school, or a business? Sitting around at home is a great way to start imagining things.
After years of marriage, my husband met a woman at a new job and dumped me for her. I didn't see it coming. I trusted him. No matter how I look back on it, I wouldn't have done anything differently as far as trust goes. Any marriage can break up for similar reasons. There's no insurance you can get. It happens. Being mistrustful just guarantees to make it happen, and sooner.

Why can't you trust him at work? You can spend a lot of money trying to prove he is cheating, or you can ask yourself why you are with someone you don't trust, or why you can't trust him.

Are you doing anything NOW to confirm your suspicions?

I agree, why can't you trust him, what makes you think there is a issue. At work, how or what do you think he is doing.