Sorry to hear about your cat. I hope he's OK now.

It was really unfortunate because mine was blocked after an otherwise routine surgery, after which the vet warned us that it might take him longer to go to the litter box! So at first we thought nothing of it and thought him not having peed yet was normal post-surgery recovery.

But I did learn to recognize the signs after that, since they kept sending him home while he was still blocked. In general, the potential signs of a blockage are pretty much the same as potential signs of infection or other urinary issues. I suggest watching/listening for the following:

* Frequent trips to the box. If a cat keeps going in repeatedly, something might be up. They either can't get pee out, or have some irritation that is making them need like they need to keep going to the box. This can start with just going a bit more frequently than you're used to and escalate to stepping in and out of the box every few minutes. Note that this can also be a sign of constipation, which can also be bad (but tends to be less critical than urinary blockage in the beginning). Usually cats have a different stance when they pee vs poop, so you can kind of tell what they're trying to do if you pay attention. Sometimes, if they keep straining to pee and can't, they might actually resort to trying to poop instead, which won't help and just keep them coming back to the box.

* Low or no volume. These frequent trips, in case of blockage, will result in no pee coming out, or maybe just drops. This _could_ also mean that the cat simply has no pee left in them and it is some other urinary tract irritation causing them to feel like they keep having to go, but that is really only safe to assume if you've _seen_ them pee a good amount recently. Usually when they are just irritated, they'll still get _something_ out when they go back in. If when you get home all you find is abnormally tiny clumps or drops, it can be a sign that something is off.

* Vocalization. If they are vocalizing more than usual, especially in or around the box. It can be a sign of pain (this sounds like what you used to catch it, which is great).

* Blood in the urine. I know it can be hard to see with many litters, but sometimes you can spot blood either in the urine, drops on or around the box, or even on the cat's fur after they go. This is more a sign of bladder irritation than blockage, but the two often go hand in hand, and one can progress to the other, so it's time to see a vet.

* Trying to go outside of the box. If they are in pain or unable to pee in the box, they may resort to trying to find another spot. If you find your cat peeing or trying to pee outside of the box when they're otherwise litter trained, something might be wrong.

* Repeated licking of nether regions, especially coupled with one or more of the above. If they are in pain or irritated, they may keep licking and fussing with the area.

* Eventually, not going to the box at all. The bladder gets distended, and nerves are damaged. This is very dangerous. The bladder is full, but the cat loses the ability to feel when they need to pee. My vets have said that as a rule of thumb, if a cat does not pee at least once in 24 hours, to get to the emergency vet asap.

There can also be cat-specific, individual signs of irritation or pain that you'd know best as the owner. For example, I noticed when my cat has trouble comfortably peeing he will start drinking a lot more water. I am not sure if this is common for all cats, but it's one thing I keep an eye on: if he keeps going between the box and the water bowl, I know something is up.

In general, a lot of it is quite individual. Cats do sometimes just change up their pee schedule. I might come home and find that mine hasn't peed at all when he normally would, but that doesn't mean I need to rush to the emergency just based on that one thing. I judge by how he looks, how he's acting, and just keep an extra eye out for any additional signs (and pay attention for when he pees next).

We can also learn to feel the fullness of the bladder through the abdomen, and many cat owners (especially those dealing with disabled cats) do. Apparently it's quite easy, and my vet would do this pretty regularly when needed, but I unfortunately never managed to get the hang of it myself (though I still try sometimes if I suspect something is wrong).