My husband passed away last year at age 42. He may or may not have a 401k, but I have no way of knowing. It was a controlling relationship and he had all that information. We separated right before he passed away, and his sister took all of the important papers. I have contacted 2 employers... one he had no 401k, and still waiting on a call back from the other employer. However, how can I check if he had any with former employers? Some are out of business.

Did you ask your sister-in-law for the papers? Did he have a will? You can check with the local courthouse and see if a will is being probated. You can check with the hospital and get a complete copy of his medical records to see what information was given at his time of death. You will need his Death Certificate and your marriage license which is usually obtained at the local Public Health/where you were married if not there you can check online and find out where you can get it.
Some employers also offer life insurance policies. Condolences to you and your family it is difficult enough without upsets. Good luck

Do you know his SSN? You can contact the SSA to see if there are any retirement benefits he or his benficiaries are entitled to.

I got shunned by his whole family because we had separated, and they have made it clear that they won't give me any papers that they might have. I do have a copy of my marriage certificate, and I already get Soc. Sec. for my 2 young sons. I just don't know where to go to check on any 401k that he had.

You've done the right thing by calling former employers. You should be aware that if he does have a 401(k) he would have specified a beneficiary who would receive the assets upon his death. So what you need to do for each former employer that had a 401(k) plan available is find out who the plan administrator is (typically it's an investmengt company of insurance company - like Fidelity or Prudential), and then you contact that administartor with your husband's SS number and inquire into whether he has an account, and if so find out whether you are the beneficiary in that account.

By the way, as the man's wife in most states (maybe all?) you and his children are entitled to inherit his estate assets, UNLESS there is a will that specifies otherwise, and even then you as the spouse are entitled to a minimum amount UNLESS you had signed a pre-nup. If your husband had assets when he died, you may want to consider hiring a lawyer to protect your interests.