2 very easy ways for them to know if the units were Army or Marine.

1. Marine Helicopters are painted a light grey. They were frequently called grey witches. Army helicopters are green and stick out like a sore thumb when flying. Also Marine attack helicopters are Cobras, noone can mistake them for an Apache.

2. Marine convoys were made with with 7 ton trucks and multiple hummers. The army ones only used hummers as lead and trail vehicles and the biggest vehicles they used were 5 tons.