There's huge custom fighters off/on button on top. Between rules and voice chat
That turns off customs as a whole (equipment+custom moves) rather than just equipment.
If you want to tell whether someone has equipment, look at the % they take and deal out. Greninja's fsmash regularly does 14%. If the person uses Greninja and their uncharged fsmash does 19%, then you'll know they have equipment. Having more attack equipment on means defense is and unless properly balanced out, they'll take more damage as well. You'll have to know how much damage moves do and if you find moves doing more damage then normal, something is up. For speed, it's something you'll have to note by observing. Is the character a little too quickly than you would think? Also, speed reduces offensive power so if you're taking less damage per hit then they should be doing, it's equipment. For balanced sets, it's a little harder but it should always be noticeable numerially %-wise.