My estranged husband died December 2014, at time of death he was living with a woman described on his death certificate as his companion. I need to find out if he had any 401 k's or life insurance. Can you tell me how I can go about this. He lived in the state of Florida, I reside in SC.

Is there a will? If so, who is executor? Are you just "estranged" or divorced? You will need Power of Attorney to get anywhere.

Since his power of attorney would end at his death, unless you are listed on those, it will depend. He will normally have listed a beneficiary that will receive those at his death. If none is listed, they go to his estate. You will need to see about the probate in his state, and see if you are considered a heir.

How did you get to see his death certificate, or even just what it said?

That person should be the one to tell you about a will or not, and any policies or not, and who the beneficiary is.
If the companion is unwilling to tell you anything, then you have a lot of work to do. Start with the probate court nearest to where he died.
Some people never find the administrators of any 401K. You need to know where he worked, and so on... very difficult.
Insurance is hard too, unless you are listed on it. There's no big central pool of all insurance to contact.

Social Security will contact you about any of his benefit you are entitled to.

As noted, both 401(k) accounts and life insurance policies list a specific person as beneficiary. So it would be up to the administrators of those (the company where he worked or the insurance carriers) to locate the beneficiary for payment. On his 401(k), if you were not legally divorced, then he could not have removed you as his beneficiary without your consent. You can contact any of his former employers to see if there is a 401(k) balance. Life insurance will be harder to locate, so I would just make sure that you can be found.

If the decedent died intestate (without a will), domiciled in Florida, and married to OP, his estate would be probated in Florida. As his widow, OP is qualified to apply to be personal representative (even though a non-resident). She should hire an attorney there to open the estate. Once appointed PR, she will have much greater ability to ask questions of companies which may hold 401Ks and the like in decedent's name. She could also depose (take the deposition of) the "companion".