The core metrics are:
* Cost variance: current period, cumulative to date, and at completion
* Revenue Variance (if seller of services): current period, cumulative to date, and at completion
* Profit Margin Variance (if seller of services): current period, cumulative to date, and at completion
* Schedule variance: current period, cumulative to date, and at completion
* Customer Satisfaction Ratings
* Aged Billed AR
* Unbilled AR
* Invoice Variances (if seller of services)
* Days Sales Outstanding (if seller of services)
* Unfilled Team Requisitions, Aged, and impact to Revenue / Profit (if seller of services) and costs
* Deliverables Completion Tracking, Verification and Validation, and Variances
* Quality Assurance and Quality Control Metrics
There are other signals you could identify but they will likely roll-up to one of these.