This specifically concerns Right to Know Law in the State of Pennsylvania
Under PA Right to Know law, could a person ask the local police department what records or information they have on a former resident of the town?
The law is here, for your reading. It is not clear from the statute how they would treat metadata requests (a question about the nature of existing records). The basic requirement of the law is in §301 ff
(a) Requirement. — A Commonwealth agency shall provide public records in accordance with this act.
(b) Prohibition. — A Commonwealth agency may not deny a requester access to a public record due to the intended use of the public record by the requester unless otherwise provided by law.
with analogous language for other layers of government. If there is a record stating what information they have on person, you can request it. But whether or not the police have any (official, open) information on William Smith is not itself a record, at least until it is made on. If you simply request all public records on William Smith, they should give you that information, but if you ask them to create a new record stating what information they have on William Smith, they may turn you down pursuant to § 705:
When responding to a request for access, an agency shall not be required to create a record which does not currently exist or to compile, maintain, format or organize a record in a manner in which the agency does not currently compile, maintain, format or organize the record.
The agency web page says "Make sure when you file a RTK request, you are seeking records and not just asking questions. The law governs the release of records, not answering questions". Obviously, it matters what a "record" is (from the definitions section of the law, emphasis added):
Information, regardless of physical form or characteristics, that documents a transaction or activity of an agency and that is created, received or retained pursuant to law or in connection with a transaction, business or activity of the agency.
"Information" is not defined under the law, nor is it a well-defined concept. It is, however, broader than "a single file". Suppose there is information pertaining to Smith in 4 different physical file folders; then those words and pictures about Smith are information, and we don't have to care what the physical form is: it's still a record. Nevertheless, they may look at a record as a discrete and countable thing.
You could ask the open records office whether "all information pertaining to a subject" is legally "a record". If it is, then you can request all information on William Smith, and that will tell you "what information they have about William Smith". See this on advisory opinions, and look here for a list of the issued and denied opinions. It needs to be "legally clear", and about the law: they seem to not want to give opinions unless they absolutely have to.