Is the minecraft mod "too many items" detectable by servers, is it even client side?
TMI, NEI, Inventory Tweaks, and other mods that add inventory management and recipe viewers work on servers just fine, and they're not detectable. The major caveat is that any feature that gives you items and blocks simply won't work, because getting items is handle by the server and not the client.
Displaying recipes, showing chunk borders and mob spawning locations, rearranging your inventory are all things that can be done just by the client, so those features generally work. Do note that there are plugins that watch your inventory for "too fast" actions though, so using any sorting features of client-side mods (TMI doesn't, but there are others that do) can be detected, if the server operator cares enough.