There’s several signs during startup that shows that a sponge server is loading.
The first sign is FML finding the coremod
[main/INFO] [FML]: Loading tweaker org.spongepowered.asm.launch.MixinTweaker from sponge-1.8-11.14.0.1290-SNAPSHOT.jar
You can also see the mixins loading e.g
[main/INFO] [mixin]: Mixing server.MixinMinecraftServer into net.minecraft.server.MinecraftServer
You know sponge is loaded when FML makes a dummy connection and lists SpongeAPIMod
[Server thread/INFO] [FML]: Attempting connection with missing mods [mcp, FML, Forge, SpongeAPIMod] at CLIENT
[Server thread/INFO] [FML]: Attempting connection with missing mods [mcp, FML, Forge, SpongeAPIMod] at SERVER
There currently is no logging messages from sponge directly.