I'm asking because I am writing my own mods and wanted to know if a server owner can parse the mod list during connection to the server and act consequently if some are missing.
As detailed in wiki.vg the Forge Mod Loader Client actually sends the list of installed mods during handshake phase inside the ModList
packet.
ModList
packet is for. You won't need to write custom logic for it. – Pokechu22 May 12 '16 at 17:21