As you seemed to intuit, you cannot sign your mother's name after her death"”and doing so might raise some legal complications that you'd rather avoid.
can cash the check depends on how you and your mother owned the oil well profits and how the checks are made out.
If you owned the property as joint tenants, for example, then her share of the land passed directly to you at her death. This is a typical arrangement for property that is jointly owned by family members"”and would likely also apply to the oil well profits, unless you signed some separate agreement for them. And if the check is specifically made out in the names of you OR your mother, then only one person needs to sign the check to negotiate it"”and that would be you.
Whatever the specifics of the property ownership and check negotiation posture in your case, however, your mother's name should now be removed from the mix. That is the job of the executor of her estate. If you are the executor, be assured that the legwork and paperwork should not be too onerous.
Best to contact the local registrar or title company to change the name on the property title and consult the title documents for the oil rights. Many companies supply their own forms to complete, which can help expedite the process.