Generally speaking, more than 1.25 psi of backpressure at idle, or more than 3 psi at 2,000 rpm tells you there's an exhaust restriction.

This tool can be used at the upstream O2 sensor port, just above the catalytic converter. Remove the O2 sensor, secure it away from any moving parts, then install the gauge. This will trigger a check engine light, but it should clear itself after the sensor is re-installed and driven a few miles.