The IND circuit from the alternator passes through the engine-bay fuse box [water ingress?] to the BSI.
The dash light is a slave function of the BSI and the "illuminate" signal is passed through the VAN to the instrument pack.
The furthest that you can go without scanning equipment is to check wire 104 entering the BSI on terminal 11 is an earth with the engine off and a live with it running.
Frankly; a scan of first the BSI and then the instrument pod is by far the best way to proceed. If the warning light status is showing as "on" - but it isn't, then a pod fault is indicated.
Only if the BSI isn't receiving the alternator signal should you start investigating manually; it's quite easy to make things a lot worse.