I usually use the grolsch style bottles with the resealable tops, but did my first bottling with crowns. Things seem sealed, but I worry about a weak seal. Would it be obvious if there isn’t a good enough seal? And is there any good way to check?
Are you using a bench capper or one of the hand-held wing cappers? I’ve never had a poor seal or leaking cap with my bench capper.
You could put some tight fitting balloons over the top of a few bottles. If they start inflating, you have bad seals.
I used a hand-held wing capper.
They can be a little inconsistent, especially if they’ve been used a bunch. Take a look at the cap after you cap it and see if it looks consistent all the way around or if it seems a little skewed to one side or another.
I’ve never had an issue with a good seal in well over ten years using a hand-held capper.
As long as the caps are seated correctly, they should seal.
If it’s not crimped properly, it should be visible.
Every time I stumble across this thread, a walrus at a Tupperware party keeps popping into my head 
I would think submerging them, upright, in water (warmer than the beer) would cause bubbles to leak from the cap if it was leaky.
Hundreds and hundreds of bottles sealed with a wing capper.
Zero fails.
here to. Also this last capper I got tends to crimp the caps slightly imperfectly and still no bad seals
I was cellarmaster at my club’s last comp, almost 600 entries and I saw just about every one of them. Some of them looked like they used spent caps that they flattened with a big hammer and used a crowbar to get the caps on, just gnarly. But they still sealed.
Seems like a RDWHAHB type of question. 
I’ve used a hand held caper for 14 years and never had a problem.
Paul
After a few thousand caps used, I’d say crown cap seals are pretty idiot proof.