You will not have any idea whether a Christian is praying for you or not unless that Christian tells you so.
There's no real way to tell, otherwise, since prayer is merely magical thinking done primarily for the benefit of the person doing the praying--either to feel like they're doing something when they can't or don't want to do anything, or to assure themselves of their superiority to the person they say they're praying for.
Corollary: If a Christian must tell you that they are praying for you, that is an admission that prayer doesn't do anything in the real world. If prayer did anything, nobody would need to tell anybody that it was happening.
Posts that touch on prayer:
The Power of Prayer, Part One. A pastor dies of cancer, and Cas learns about a big secret Biff's carrying.
The Power of Prayer, Part Two. Close to deconversion, Cas consults the Bible to get answers.
Prayer Warriors for Jesus. Biff starts a college club, and then loses it. Cas realizes something about prayer that she will never be able to un-see.
Magical Thinking. Why prayer seems like such a great idea to Christians.
Differing Opinions, Part Two. One of us had to be wrong. We couldn't all have talked to our god.
The UYC: Praying for Reconversion. It's not that Cas thought that Biff's prayers for her reconversion would work, but those prayers were a potent indicator of things seriously wrong in the marriage and religion alike.
How Not to Fix Everything: School Prayer Edition. Christians often think this would fix everything, but it wouldn't--and shows us lots of things about Christians that they likely don't want to reveal.
The Power of Prayer in Christian Marketing. Thomas says his church plant failed because he didn't pray enough. Here's where this common Christian excuse epically fails.
The Day My Prayers Changed. Eventually, every Christian's prayers change in nature. This was when Cas' prayers changed, and how, and why.