Often bringing mindfulness to the body will reveal that we’re making too much effort. When we’re forcing ourselves to be mindful, there’s likely to be some tension, contraction, tightness, or stiffening somewhere in the body. It can be helpful from time to time to check areas of the body where we commonly hold tension - for example the small muscles around the eyes might be scrunched up, or there could be a slight frown on the forehead, or the jaw might be clamped tightly shut, or there’s a feeling of tightness in the chest and slight holding of the breath.
If you notice tension in any of these areas, invite it to release, to relax, then gently return to the breath. If there’s some residue of tension that doesn’t relax, that’s ok. Don’t try to force it, as that would be counter-productive. Instead, try to just accept that it’s there in the background, and continue with your meditation.
A second way to recognize forced effort is to notice if there are any thoughts of judgment in the mind as you return your attention to the breath. If you’re able to simply notice that the mind has wandered and then come back to the breath, this usually doesn’t take much effort or feel particularly forced. But if there is any background sense of “Oh, no, wandering again. How many times has it been now? This is hopeless. Get back to the breath, dammit!” then that extra layer of judgement can create a sense of needing to force the process.