Read recently (maybe even on this Forum) that a lot of Asthma Fatalities are caused by Asthmatics underestimating the severity of an attack they are suffering.
I decided to supplement my 'gut feeling' as to how ill I was by incorporating some more objective criteria. I guess knowing how you feel must remain the main criteria but other stuff you can consider are as below. (Note, it's possibly best to make decisions as to what to do next based on two or more criteria rather than relying on just one. That said, I'm not a Medical Professional, so form your own view).
a) Fingertip Pulse Oximeters (that thing the Doctor or Nurse puts on your finger).
Although I'm in the U.K. I managed to get one from Amazon.com as the the exact same model appears not to be sold on Amazon.co.uk (It cost $24.99 Dollars, about £30 including shipping from the USA). It claims to be accurate to 1% rather than the 2% that many other models mention. Note that market leader models costs a 'great deal more', but if - as claimed - they are the type used by medical professionals, then it could be a sound investment. My one is called the Acc U Rate premium fingertip pulse Oximeter. It seems good and is from an American Company that - if you buy direct from - costs a lot more money for shipping, so via Amazon.com seems the cheapest option.
There's plenty of info about these gizmos online if you need more details. Essentially, if your rating over a five minute period is 92 or lower, it seems you need to be thinking about heading for the hospital. If the rating is 93 to 95 you're suppose to monitor the situation. If 96 or above, this Device can be disregarded.
2. Peak Flow - Too poorly to use it, or reading way below your normal reading.
I have a Peak Flow Meter at home, got mine from Asda Supermarket Pharmacist for about £12
3. Ability to hold a conversation without struggling to breath
4. How you feel when you climb stairs or do some other activity you can normally cope with without issue
5. Frequency and Dosage Size of your Medication
There's probably some good ones I've failed to mention (including having your own spirometer at home) but - a couple / few of these factors taken together can help you (a family member or a Carer) make a more informed / objective decision as to when to phone for help or go to the Hospital. Also, by being able to tell a Health Professional that you made your decision to call based on whatever set of criteria you decide is right for you, you'll (hopefully) help them to decide more effectively on what action needs to be taken.