What are the signs that asthma is getting worse?

Difficulties. Difficulty breathing, increased cough, increased wheezing, trouble sleeping or cough at night, decreased exercise tolerance (you can't do as much), and so on. If you measure your breath, you may have a decrease in the amount you can forcibly breath out. If you have pulmonary function testing done, it shows more airway obstruction (usually). More importantly, you feel worse.
More... ...Nighttime awakenings with shortness of breath, decreased exercise tolerance, and especially an increase in the need for rescue medications.