How difficult would it be to prove slander at work and be sued successfully?

A co-worker who works my shift is strongly suspected of theft as it always happens on his shift. I have stated my reasons to others why I believe it is him and others knows he does not do his job properly. He is a security guard who leaves loop holes that could be excuses to cover his actions. He has others complain of his poor work, and now with fingers pointing at him, he is accusing me of slander although he has said things about others also. If I were to be fired over stating my opinion only, would that be wrongful dismissal?

many co-workers are becoming frustrated and unhappy knowing he is sleeping, not doing his job, and I do have proof of that, plus his schedules match to the times of the theft...it is hard to replace guards, so boss keeps him along with the hard working guards in fear of being short a worker...he is also letting unauthorized employees into the building where thefts have occured.

Truth is an absolute defense against slander which means if you are telling the truth, you cannot be sued for slander. However, it sounds as if the truth of your allegations has not yet been proven. Learn from this & keep your mouth shut. Don't gossip no matter how tempting. If you want upward mobility, gossipers generally lose out on that front. If you are management, all the worse that you'd be bad mouthing employees except within the confines of a management meeting where an employee's conduct is officially under discussion.

Whether losing your job over it would be wrongful dismissal depends on whether you work in a right-to-work state or an at-will state. Most states are at-will states & you can be fired for anything except being a member of a protected class. So, in no state can they fire you for your race, gender, religious affilliation, sexual preference, or age. I believe a couple new categories may have been added, but no boss would ever walk up to an employee and say "You are fired because you are a woman" & then note it in the employee's personnel file - so proving it would pretty much be impossible.

If you suspected the security guard, you should have reported your suspicions to your supervisor but ONLY if you had some serious foundation for your suspicions such as he props doors open that are supposed to be kept locked, that you knew there were 10 boxes of pens when you went into the supply cabinet at 9:15 a.m. and when you went back at 9:17 you saw the security guard leaving with a big bulge under his jacket and there was only one partial box of pens left. something fairly substantial and particularly if there are other employees who have seen similar behavior - maybe several of you saw the propped open door on several occasions and a couple of you saw him leaving the supply cabinet with the big bulge under his jacket.

For one thing you should never accuse anyone of anything based on circumstansial evidence. You handled it completly wrong. I would recommend you apologize to him. Can you be sued for slander? Yes if he has proof of it and it sounds like he has. If your dismissed it won't state that you stated your opinion it will be disruption, or troublemaker. You should of kept your opinion to yourself while looking for solid proof. Do you have a human resource department? I would recommend going to them and admit you made a mistake and asking them for help.