Disinherited heirs at law.
In general, any person who would be entitled to receive a share of the Testator's estate, but for the existence of the Testator's will (meaning the Testator's "heirs at law"), should have sufficient standing to challenge the Testator's will. For example, if the decedent is survived by three children but only two are provided for in the Testator's will, then the third child should have legal standing to file a will contest. But some words of caution - if the Testator had an older will that also cut out the third child, then the third child will have to knock out the older will and any wills prior to that where the third child is cut out in order to get to the point where the Testator is deemed to have died "intestate," or without a will.
Beneficiaries and fiduciaries named in a prior will.
Any person or entity, such as a charity, named in an older will signed by the Testator that has been cut out of, or whose share has been reduced, in the Testator's later will that is being offered or has already been admitted to probate should have sufficient standing to challenge the later will. But the same words of caution mentioned above also apply to these beneficiaries. In addition, any person or entity, such as a bank or a trust company, named as a fiduciary in the Testator's prior will that has been replaced in the Testator's most recent will should have adequate standing to challenge the later will.
Who can't file a will contest?
The bottom line is that not everyone involved in the Testator's life will have the appropriate legal standing to challenge the validity of the Testator's Last Will and Testament. So even if you suspect that a loved one's or friend's will is invalid, if you weren't named as a beneficiary in an older will or you're not an heir at law, then you won't have any legal standing to file a will contest.
If you're still not sure about your legal rights with regard to challenging the probate of a loved one's will, then you should consult with an estate lawyer to determine if you have the appropriate legal standing to file a will contest.