The details of how an estate is handled varies from state to state (including the name of the court that handles these matters), but generally there is enough information available to the public for basic information about an inheritance to be researched (assuming the decedent didn't have a living trust that hid the details of the estate's assets and/or distributions).
If the story is set in Colorado, I believe the state has adopted the Uniform Probate Code. That might complicate matters, when it comes to the availability of detailed public information, because under the UPC, if the estate is handled without any objections by interested parties, a lot of information can be kept private (but doesn't NEED to be kept private, so you could go ahead and have it be public information if necessary for the story).
If there was some sort of will contest, then more information would likely be publicly available, because the inventory would have to be filed (showing what assets the estate owned on the date of death), and a contest would likely end up requiring a formal closing, which requires the filing of a detailed account (showing who got what), whereas with a simpler estate, the account can be sent only to parties and not filed with the court where it would be public.
Generally, the will itself and the names of all the relatives and any non-relative beneficiaries of the will are also public record, so even if the account wasn't public record, an investigator could make an educated guess about the value of the inheritance, from reading the will and the inventory.
Not giving individual legal advice, just general information. There is a fairly detailed website for t he Colorado probate courts, if you search the web. Unfortunately, the one place where I saw a reference to FAQs for estates, the link either didn't exist or was broken. There are some PDFs, though, that offer basic information on filing a petition and doing the closing statement, which are the two documents that would have the information your researcher would be looking for.