How can I tell if I'm singing on pitch? Obviously, my first answer is always come to me for help since I'm a vocal coach! (I teach online in addition to in Los Angeles). In addition, my advice is to consider a tuner application. There are many with free versions you can download easily right onto your phone. These apps also record your voice so you can listen back to develop an ear for noting the difference.
I love this one - Perfect Pitch -because it includes exercises and a pitch detector which lets you know if you are above or below the note. Check it out!