What Operator wrote was correct in the sense that blindly guessing on proper tension with no experience, no GUIDANCE and no working example isn't the most constructive way to learn. Having a properly tensioned example wheel helps.
But there are clues and hints that can help you if you're forced to just deal with a solitary wheel. One method of judging when you are getting close to over tensioning is by the feel of the spoke nipple as you turn it. If you have properly lubed spoke threads, the nipple, and the rim's nipple socket, you have a good base for judgment. If it gets quite hard to turn, the spoke twists a half turn, moving in unison with the nipple, and you're hearing harsh creeks, then you're probably getting too tight.
If you're building up a wheel, things are going fine and progressing steadily, and in a later round of tensioning, you start to find the wheel is going out of true, contrary to the way you're intending it too, then the wheel is too tight. These clues help when you're in the actual process of tensioning a wheel. There are other clues that indicate improper tension in an already built wheel.

A tension meter helps, and is a great learning tool. If you choose to buy one, use it correctly, pay attention without turning it into a crutch. You can use it to build a wheel correct according to the numbers, paying attention to how the tension has changed with each round of truing and tensioning. Build on this experience by creating a wheel without it, attempting to replicate the final tension of the first wheel, then use the tension meter as a final check. If you are patient, pay attention, and use proper techniques, chances are you'll get to the point where checking your wheel with the tension meter starts to seem unnecessary. But as long as you have the tool, it's a good idea to check yourself against it occasionally. By this time, you're hands or ears will have developed the experience, so you can pick up any built wheel and be a good judge of it's proper or improper tension.