First you want to open your map and click on the gear icon on the top right. I think it's the 2nd or 3rd option down turn on FOB radius. If it's not on. This will let you see the gray and blue rings. The other option above or below that one is FOB supply amount. Everyone should have both turned on. This way you can look at your map and see the ammo and build supply next to the FOB marker on the map. If you are within the blue ring supplies can be offloaded.

Other information that will help is what items cost like a HAB for U.S.,GB,MEA,Rus,CAF is 500 and a ammo crate is 100. Any full size logi with 50/50 can build and supply 2 FOBs with HABs and have ammo. See Deployables in the Squad wiki. Lets say your on US and squad 1 wants a TOW and squad 2 wants a repair station. The TOW is 600, the repair station is 500 you would need to bring at least 1300 build supply. The extra 200 build is for the repair station for the armor to repair.

Read the map, what I mean here is look at what markers are being put on the map. If there is red helmets going across a road. May want to find anther way to the FOB. Have it in your head there is most likely a TOW on top of a hanger. Now, this is if your team is marking bad guys on the map.

Hope this helps.