Everybody is different, so you need to work out at what point you are dealing with "more than just a cold."

Here are my personal signs:
Short of breath even after using levalbuterol inhaler or neb.
Coughing hard enough to hurt ribs or throat or leave me breathless.
Colored mucus - either from lungs or sinuses, anything darker than very pale yellow I watch carefully. Green is my "danger sign" - that's when I know I need antibiotics.
Extreme fatigue - bone tired after a full night of sleep, falling asleep when I sit down to read.
Fever over 100 for more than one day (my usual temp is between 97.7 and 98.0).

For five years before my MAC diagnosis (I'm sure I already had it), I went through this cycle several times each year, and this plan was developed long ago, my pulmonologist is on board with it.

If I experience the cough and my cold is more than 7 days old, my personal treatment plan calls for steroids and I always have a proper amount on hand.
If I experience the colored mucus or fever, I call my doc for an Rx which she phones in to the nearest pharmacy (we travel a lot and live in 2 states seasonally.) Whoever is covering for her has my records, and I have always been able to get antibiotics. Often she asks if I don't see improvement in 3 days on antibiotics to get a chest x-ray. I haven't had it go that far since my MAC was treated in 2018 & 2019. In fact with saline & airway clearance, I have had only 3 exacerbations in 4 years, with only one ending in antibiotics.

How does everyone else recognize their critical point?

Sue