It would depend upon just how bad the clog really is. The first sign would be delayed shifts. If its a really bad clog, it wouldn't go into gear. The next thing would be a feeling like its bouncing in, and out of neutral at idle. Sometimes a clog would be felt more when its first started up before the fluid gets warm. As the fluid expands, so does whatever is clogging. You could also tell if you had a transmission pressure gauge connected. The readings would be off (some too high, and some too low). How old is your fluid? Do you regularly replace it at 30,000 miles, or more than that? Severe driving requires more frequent changes than this.
