Certainly, here are specific signs that might indicate a cracked heat exchanger within your furnace:

1. Carbon Monoxide (CO) Presence:

• CO Detector Alerts: Elevated carbon monoxide levels detected by a CO detector placed near the furnace.
• Symptoms: Experience symptoms of CO poisoning like headaches, dizziness, nausea, fatigue, confusion, or flu-like symptoms when the furnace is running.

2. Soot Accumulation:

• Visible Soot: Sooty residue or black streaks on the furnace, surrounding areas, walls, or vents.

3. Unusual Smells:

• Strange Odors: Though carbon monoxide is smellless, odourless, tasteless
  But when Heat Exchanger is faulty,clogged or crack you might have follow smells.
• Detection of unusual smells like a strong metallic or chemical odor when the furnace is in operation.

4. Flame Irregularities:

• Flame Appearance: Abnormalities in the flame when observing it through the furnace's viewing port, such as a wavering or flickering flame instead of a steady blue flame.

5. Visible Damage:

• Visual Inspection: Visible cracks, holes, or damage observed on the heat exchanger, if it's accessible for inspection.

6. Age and Heating Inefficiency:

• Older Furnace: If the furnace is quite old or nearing its expected lifespan (typically 15-20 years), it might be more prone to heat exchanger issues.
• Decreased Efficiency: Noticeable decrease in the furnace's efficiency, leading to increased utility bills or difficulty in maintaining desired temperatures.

7. Multiple Symptoms:

• Presence of several signs together, especially if they occur simultaneously when the furnace is running, might strongly indicate a cracked heat exchanger.

What to Do if You Suspect a Cracked Heat Exchanger:

• Safety First: If you suspect a cracked heat exchanger due to carbon monoxide presence or other safety concerns, evacuate the premises immediately, ventilate the area, and contact emergency services and a professional HVAC technician.
• Professional Inspection: Have a qualified HVAC technician inspect the furnace if you suspect a heat exchanger issue. They have the tools and expertise to conduct tests, examine the heat exchanger, and provide necessary recommendations for repair or replacement.

Addressing a suspected cracked heat exchanger promptly is crucial for safety reasons, as carbon monoxide exposure can be life-threatening. Regular maintenance and inspections by HVAC professionals can help identify and resolve such issues before they escalate.