Maintenance fees have to be paid at 4 years, 8 years, and 12 years, just to keep the patent alive for the full term, which used to be 17 years from issue. After that, you can't pay more fees to keep the patent alive. The US Constitution only empowers Congress to grant patents and copyrights for a limited time, and they could theoretically make it a longer period of time, but they settled on about 15-17 years as the right amount of time for patents.
You can be assured that any patent that starts with the number 4 is now expired. Also, any patents that issued prior to 1995 are either expired or about to expire. Patents under the new term calculation actually get 20 years from filing date, plus patent term adjustment for any undue delay in prosecuting the patent application to issue. The patent isn't enforceable until it issues, so if it takes 5 years to get the patent issued, then you only get a maximum or 15 years of term, unless there was undue delay.
Undue delay is when the delay was not the patentee's fault, but was the fault of the patent office. So if the art unit is backlogged, and they don't issue the next Office action in time, then undue delay occurs, and patent term adjustment begins to accumulate. Similarly, if the patentee appeals to the Board of Patent Appeals and Interferences, and it sits in the queue for several years waiting to be heard, and the BPAI overrules the Examiner and the patent issues, then all of that time it sat in the queue, minus six months, gets added on to the patent term. The same occurs if the applicant appeals to the federal circuit and waits years more.
As a result of all of these adjustments, it has become much more difficult to determine the actual patent term. The amount of patent term adjustment is supposed to be printed on the front of the patent, so that helps, but it's not always right, and it really only tells you the maximum amount of term. You still have to look up and see if the maintenance fees were paid, and there are all kinds of options to claim priority from other applications, which can change the effective filing date from the one listed on the patent, and toll the start of the term at that earlier date. Additionally, there could have been a terminal disclaimer filed in the application to disclaim part of the patent term to overcome a double patenting rejection where the Patent Examiner says that your patent's claims are too similar to those in one of your earlier patents.
So it's possible that patents could be expired and you can't tell without doing some research. It can be difficult for patentees to figure out their own patent terms. There was an example recently where the patentee spent a fortune getting a patent with appeals and such, and then paid the issue fee when they finally got an allowance, but it turned out their patent only had a term of about 12 days due to some complicated issues.
Here's the best part:
This is Not Legal Advice
I Am Not Your Lawyer