So, assuming you're willing to play a little doctor...

 - Take their temperature. This is intimidating for some, since it has to be done rectally, but the temperature of a cat should be between 99° and 120.5° Fahrenheit. 
 - Check their gums. Gums should look pink and healthy, but pigmented black and brown areas are not a sign of illness, look for a non-pigmented area.
 - Check responsiveness of blood flow. Basically you can lift their upper lip and *gently* squeeze a bit, the white mark should fade back to pink within 2 seconds, if it's longer, contact the vet. You can see the effect on your own skin to get a sense of what you're looking for.
 - Dehydration check with the scruff of their neck. If all is well, a pulled scruff should spring back into place when released.
 - Check the heart rate. Tricky and a little hard to master, place your hand on their left side behind the point of their front elbow, but a cat should have a heart rate between 120 and 240 bpm. You can use a stethoscope if you have one, they're not that hard to get and probably better.
 - Check their breathing rate. Cats should be about 10 to 40 times a minute depending on rest level. Also, check to see if it seems labored and, unless it sounds normal, check with a vet.

Any of the above, really, are very good reasons to take your cat to the vet. 

If none of these seem prevalent in any of the cats, then some isolation time is probably needed. Ideally, you'll have sufficient litter boxes and rooms for this, but isolate the cats from each other for 24-48 hours and try to detect the one with the issue based on the isolation. 

If you can't do any of that, then bundle them all up and take them in to the vet. We did that once with 7 guinea pigs just to get them all sexed properly (it's surprisingly hard unless they're pregnant) and found out what we needed.