Did you receive the Spirit by works of the law or by hearing with faith?
Paul doesn't seem to think that this is a hard question. He obviously expects the Galatians to be aware at once that the Spirit came upon them as they heard and believed the good news about Jesus. It's foundational to his argument, not only that this was the case in the past, but that it continues to be the case in the present - does he who supplies the Spirit to you and works miracles among you do so by works of the law or by hearing with faith? He clearly expects that the Galatians will be able to give a straightforward and unequivocal answer: the Spirit is communicated to us, and works powerfully amongst us, as we hear and believe the message.
What do we do when the answer to that question no longer seems obvious?
If Paul hasn't massively misjudged the Galatian Christians - if they are in fact able to provide the answers which he expects to these questions - then it becomes baffling that they would be looking to work out their day to day godliness by way of the law. And indeed, Paul seems pretty baffled and perplexed throughout the letter. Obviously they don't see the law keeping which they are considering adopting as contrary to faith, and don't see clearly as Paul does that working out your holiness by the route of law is incompatible with reliance on the Spirit. But at least they know, or should know, that it is the Spirit, received as they've heard and believed the gospel, who has provided the energy of their holiness thus far.
What if we're not even sure of that?
It seems pretty clear that spiritual experience is not an optional extra in the Christian life for Paul. If you can't testify that you received the Spirit when you believed, and that the same Spirit continues to be poured out in your church community as you gather around the gospel with faith, then of course you will start to look around for another way to power the holiness engine. But the engine of genuine godliness only runs on the Holy Spirit. If you pour your own efforts into that fuel tank, whether shaped by the law of Moses or any other scheme, it will break down your Christian life, as surely as filling my diesel car with unleaded will lead to going nowhere fast. It is the Spirit or nothing.
At this point, it's easy to get caught in sort of meta-law. I can't get holy by my own efforts, I need the Spirit - now, what technique or discipline can I follow that will ensure that I experience the Spirit's power? How do I do it? How do I do it? Bang, you're keeping the law, you're holiness engine blows up.
I can only think that the answer is, at least in part, waiting. We have a hope of righteousness - certain, because grounded in Christ, but only very uncertainly worked out in our experience. Wait for it - that active waiting which involves prayer and faith and watching for God's work.