The most basic setup you and a helper might try without a meter is to use some software to generate a sine wave tone at the sub(s)/mains crossover frequency and feed that into your system, likely in one of the stereo modes. The idea is that, if all is in perfect balance between your sub(s) and your mains, both the mains and the sub(s) will be contributing an equal amount of output at the crossover frequency. With one person being the listener and the other person being the switchover tech, alternate listening to the crossover frequency tone from the mains without the sub(s), then the sub(s) without the mains. Go back and forth with as little silence between sub(s)-listening and mains-listening as possible, adjusting the subwoofer level until the output of the sub(s) and mains seem to be practically the same at the crossover frequency for most of the prime listening area. If you experiment with sub placement, the level comparison will have to be repeated for each arrangement you try. I suggest two persons so that the listener does not have to lose the level impression they have between listening samples. Of course the two people can swap roles for a time to confirm your settings.

As further confirmation, listen to a recording that has one or more single, non-processed instruments with frequency components from those instruments that span both the sub(s) and the mains. In other words, you are not comparing the sound of a bass to the sound of violins, you paying attention only to instruments that have both low-bass and mid-bass/treble coming from the same instrument. Do those instruments sound natural and balanced regardless of what other instruments may be playing?

Definitely think about an SPL meter for the future.