I found an old un-cashed check dated 1994 for $487 from my Xerox Retirement plan with no expiration date. Can I still cash it?
Checks (in the US, anyway) are only good for six months after they have been written. After that. under the US Uniform Commerical Code they are considered "stale checks" and banks need not accept them. My experience is that they generally won't -- but you probably shouldn't count on that, either when figuring out whether to try depositing an old check or figuring out how much cash you need to keep in your checking account to cover recent stale checks.
The check you now hold is certainly a statement of intent to pay you and thus is a useful document to supplement other evidence that they still owe you the money -- but since checks can be cancelled and/or a replacement check may have been issued, its value for that purpose may be limited.
You can try depositing it and see what happens. If that doesn't work (or you don't want to bother trying it) you can contact the retirement plan, point out that this check went uncashed, and ask them to send you a replacement. If they haven't already done so (you might want to check your own records for that), there shouldn't be any problem with this.
(Note: Many business checks have a statement printed on them that they're only good for 90 days or so. If yours does, you can skip trying to cash it; just contact the retirement plan offices.)
In California, dormant accounts, including uncashed insurance and dividend checks, escheat to the State and can be claimed from the Custodian for unclaimed property. There is no time limit. My wife claimed a years-old stock dividend this way.
This assumes that the money is still owed. If the retirement account is still active, or was closed/rolled over with the entire balance cashed out, it really doesn't make much difference since you either still have or already received the amount of the uncashed check.
Your jurisdiction probably has something similar.