Bottom line, if a singer can't tell if they are matching a pitch or not, they need to consider landscaping.

And don't DARE consider correction tools or the whole world will scream "AUTOTUNE" and send your soul to burn in hell for eternity.

Though somehow EVERY other tool is acceptable, and nobody can really define what they think autotune really does. It has be come en vogue to scream AUTOTUNE, stamp a red A on the chest of the singer, and ostracize them. Meanwhile those same people use harmonizers and such all the time.

Still comes down to raw talent. Either you can sing in pitch or you can't. Lots of practice can help but there still needs to be the native ability present. Lessons can improve range, power, breathing, phrasing... but natural ability, no. If someone can't tell when they are a few cents flat or sharp.... see "landscaping" above.