When you have mods(Modifications) to Minecraft, are people going to know like Admins, Operators, or even other players? Will the server be able to detect them?
Usually, the answer would be a solid no. In almost every case, the server cannot directly detect the presence of a mod. There are some mods that are specifically made to be recognizable to the server, but those are few, and even then, they can be modified to disable said functionality.
However, this does not mean that you are safe. Though servers cannot detect mods directly, they can sometimes detect the effects of them. For example, the NoCheat Bukkit plugin will detect if a player does certain things that a player is not supposed to be able to do, like fly, run super fast, place blocks in mid air, etc. Mods like this that are "cheat" mods can in some instances be detected by the server by their effects.
On the other hand, some mods will never be able to be detected by the server. For example, minimap mods don't change gameplay, so those don't have detectable effects. In cases like that, you are completely safe.
It does not detect the mods. Some mods are client side only and will have no affect on the server. For instance most of the MiniMap mods are Client only and work fine on servers.. texture packs, etc. Mods which alter how the game works for the player though generally will not work and results can range from what Dima said, the server glitches, to just outright crashing, usually just your client instead of the server. Take the recipe mod, if you do nothing with it the game works fine, try and craft a recipe book though (which will show up as that screen is client side) and BAM, booted from the server you will be for asking it to make something that does not exist.
Hope this helps.
PS: There are a set of server/client side mods out there that can self detect each other, but you have to set up both sides for this to work. After that any supported mods can be installed just by connecting to the server to get the rest of the mods on that server.. I forget the name of it, was for Bukkit.. Leaf maybe?
Yes for some reason the servers detect the mod but you will not be able to use it if the mod is not installed on the server itself, like for example i have woodcutting mod which lets me cut down the whole tree if i only cut 1 block of it but when i get on my friends server the tree just glitches out for me because he does not have the mod but i do. The other players wont be able to detect if you have the mod unless it something that glitches everything around you.
There is a server that specifically states no flymods or xray mods, meanwhile i have that plus more, i just don't use them on servers, they're more for aid in designing adventure maps. As long as you don't fly or act suspicious (like looking down at the ground all the time, which is a dead giveaway that you're using xray) you should be fine
It's possible to detect a few mods like TMI or forcefield, but you can only use them with the servers permission so they almost never will be detected.
Mods like pixelmon, tale of kingdoms and others do crash your client when joining a regular server because they rely on missing server-side features.