If your phone is being tapped, and it is less than a professional job, the tap might be detectable using a telephone tap detector device made for that purpose. While these are available on-line from various retailers (some reputable and some not), most commercially-available and affordable devices are not very good (either too complex for the average non-professional user or are unsophisticated, as in not very sensitive). Many are just plain junk. I would recommend against purchasing such a device and trying to detect/locate a potential telephone tap yourself. You'd probably be wasting your time and money.
You can contact your local telephone service provider and ask to have your line checked for illegal wiretap devices, but in many cases, they will just check to see if the line is working properly (not loaded down excessively). In other words, don't expect much help or sympathy from your local service provider. They are not going to expend much time and effort to ease your fears/suspicions, because of the cost involved. They would probably refer you to a private security contractor, which could be quite expensive. If you frequently discuss very sensitive information, such as business or legal data, then it might be worth it. I don't know your particular circumstances, so I can't really advise you on that.
If your line is being tapped, and the tap is professionally installed, a standard line sweep will not detect it. The best line tap devices are ultra-high impedance, which means they don't load the line, which is what a standard sweep checks for, or are inductive link devices, which mean they are not physically connected to the line (therefore cannot be detected using standard methods). Some are wireless transmitters that relay your conversation to a receiver located off your property.
There are various methods for detecting (and in some cases neutralizing) suspected tap devices, but these can be costly, time-consuming, and potentially damaging to standard devices (such as telephone units, fax machines, answering machines, computer modems, satellite receiver terminals, etc) connected to your land line. These would have to be disconnected first to avoid potential damage to them.
To summarize, even if you physically inspected your NID (Network Interface Demarcation panel), and every inch of telephone line throughout your home, it's unlikely that you would be able to detect a professionally installed telephone tap. The tap could be a radio transceiver or telephone junction box interconnection before the line even traverses your property. Professionally-installed taps are designed not to be detected or neutralized, otherwise it wouldn't be a professional job.
You should always remember that a standard telephone land line is just slightly more secure than a wireless telephone or radio broadcast. You should never say anything in a telephone conversation that you would not want to be repeated or made known publicly. And cell phone conversations are even less secure than a landline conversation.
Electronic technician; years of previous military and security experience with various electronic devices.