My ear is not very good. I can always hear when something is off but can rarely tell you if it's sharp or flat. This means i need to use position markers a LOT or I'll play off. If i keep an eye on the markers I'll play pretty well.
If you're going to play fretless, and play it well, you've simply got to have a good ear. It's essential. Do whatever you can to train your ear appropriately.
MM