Quite often, you will see some unfamiliar text where you know the distinction hasn't been made between L and £, or any of the other letters with things hanging off them or floating above them. What I tend to do is just guess. You have to be right some of the time, surely?
The frequency of usage of the letters L and £ are very close. It's not as if one occurs far more than the other or anything helpful like that. Just take a look at
this.
Someone might say something about softened sounds in Polish if they want to get really technical, but as far as I know, I don't think £ can occur before I.