You wouldn't know. A ten week old baby's cries might mean: I've pooped, I'm hungry, I'm teething, I'm bored... etc.
I found it helped me to run through a checklist in my head, and I was also compulsive obsessive about knowing when they'd last eaten/drank/shat/slept etc. I just felt that I'd then be able to make a more accurate guess of what they wanted. I also fed on demand, which often just shut them up, offering comfort or food, or both).
Even when they are 18months - 2.5 years (i.e. before they can verbalise their feelings and physical needs with some sort of accuracy), you still have to second guess whether it's just over tiredness or an ear infection etc. and you will still sometimes get it wrong. But you'll often get it right.
Re. the thermometer, if you can get a digital ear thermometer, they are quite handy - easier to use on a wriggly baby or child. If the temperature is over the guide (I still have to check, but take it 3 times in both ears (as one can be warmer if they've been lying on that side) and take an average. If it is over 37.8c - I think - then administer meds as instructed on the packaging (not sure what the doses are for very small babies, you'd need to check). If you are ever really worried, i.e. floppy, shallow breathing, unresponsive, then take to A&E or call the ambulance. This doesn't happen very often, so don't fret - you'll cross that bridge if ever you have to come to it. As a rule though, I always keep calpol and ibuprofen at home, in the store cupboard.
Don't read the silly books or articles if they make you feel inadequate or guilty. Come to MN, and be told to pour yourself a G&T and go easy on yourself. It's hard enough work already without feeling guilty all the time!