Locomotives are usually sent with horn already installed.
-Modern GE stuff, such as P42's, and P32BWH's, have K5LA's, with a few K5L's and "hybrid" K5LA's thrown in. There's been an occasion where an M5 was tossed on a Dash-8.
-Modern EMD power, such as the F59PHI's, have the same stuff. The complementary cabcars have the same. Although, there is a P5 on an AmCal cabcar down in the bay area.
-Cabbages have K5LA's, excepting one or two out there, one with an original P5a (Heartland Flyer service), and one in Cascades service.
-Switching power tends to have an older Leslie, but there's a mix of K5LA's on units such as the GP15D's and CF7's.
SDP40F's, Leslie SL-4T's. F40PH's, Early versions had Nathan P5's, later had Nathan K5LA's. Genesis series: Nathan K5LA/K5L and Various Messed up Nathan Productions. F59PHI's, K5LA.
As always, there are exceptions. Some BNSF units are known for having P3's and P5's. if you are really that concerned an experienced person uses their ears to detect what it is.