Are you are an heir or beneficiary?
A successful will contest invalidates that will, so that if there is a prior will it will then govern and, if none, then the laws of intestacy prevail. If you are not entitled to anything in a prior will or by intestacy, then you really have no standing to file a will contest.
Is it really a Will?
Wills require certain formalities, without which the document is really not a will at all. While the decedent might have intended it to be a will and called it a will, if it only creates a present interest, or lacks the required witnesses, it may be contested as a non-testamentary or failed will. Interestingly, the reverse might be true: a document which purports to transfer something (like a deed) presently, might be a disguised will which is only to take effect at death, hence a testamentary transaction without the statutory requirements.
Is there a no-contest or "in terrorem" clause?
Some will have no-contest clauses, meaning that merely by filing a will contest you will be determined to have predeceased the decedent. California has a means to obtain a declaratory judgment ahead of time if the no-contest clause will be invoked by filing your will contest and thus not jeopardize whatever interest you have.
Was there a lack of testamentary capacity?
You must examine the decedent's condition when the will was executed. They may be ill, they may be dying, but as long as they know and understand what they have and who the natural objects of their bounty are, they may do as they wish. Even people with dementia have moments of lucidity, so this issue can require considerable evidence and expert testimony.
Was there fraud?
Fraud is a big category which includes "extrinsic" fraud and breach of fiduciary duty, overlapping with issues of elder abuse. Did the decedent know and understand it was a will or think it was something else? Did the beneficiary have a hand in preparation of the will and was the beneficiary in a position of trust and confidence? These grounds are usually vigorously litigated and may be expensive to prove.