my dad died and left everything to an ex-wife, should I contest the will?
In the US we have the freedom to leave our estates to whom we wish, unlike many countries in the world. You can challenge it but unless you can prove fraud or undue influence on the part of the ex wife you're not going to win.
I am just going to add that the conditions under which you can contest a will also include incapacity, or duress. This means you would need evidence that your father was unable to understand what he was doing (incapacity) when he changed his Will (by use of medical records, etc.) or that he was in such a mental state (duress, or otherwise) that he was easily susceptible to manipulation by the ex-wife to change his will to benefit her (fraud). In general, there is nothing to be done, but if you have some facts or evidence to help you, you might want to contact a probate attorney in your area.
Well--just to add--There are some very specific statutory timelines in which you must file a will contest once the will is adminitted to probate. You must file the contest within 4 months after the will is admitted to probate or else you will be barred from contesting it. AND you must have sufficient evidence of Undue Influence, Fraud, lack of capacity, ect as discussed above. Since you posted this 5 months ago, I suspect that you may be time barred unless you posted this before the probate started OR before your dad even died. So, without more information, I can't really help. Quite frankly, contesting a will is one of the most difficult legal challenges I have seen. I have defended several of them, and it is almost impossible for someone to successfully challenge a will or trust.
Generally any interested party can contest a will. The question is, will you succeed? It is rare to have a will declared invalid, especially if the will was prepared by an attorney (as opposed to a will prepared by the testator or by any non-attorney). Also, a will contest can be a long, expensive process. Children generally have no right to prevent their parents from dis-inheriting them. Thus, a successful challenge to your father's will must establish a legal basis for declaring it invalid, such as undue influence, fraud, lack of capacity, or failure to comply with formalities regarding the will itself (it must be a written document, dated, signed by the testator, witnessed by 2 witnesses, etc.)