Even if a sign is posted and it's a "new" ramp to you (or low tide), do yourself a favor and check for yourself. A leadline, boathook, or use a piece of tubing. Most ramps have the same "slope" or angle, so I use a little trick at ramps. All it requires is a piece of tape. I use red electric tape myself. I note the waterline on the trailer as needed for launching the boat and I mark it with the tape. When the time comes to recover, I now have a good idea on how far I need to put trailer in water.
Thanks for this very nice tip. I usually put in in the spring and pull out in the fall. By fall, I usually forget just how far to back it in :-D