Easiest thing to do is to take a look at the pad. Flip the bike over and remove the wheel, and then should be able to visibly see how much pad you have left.

Just because the brake squeals does not mean it is time to change the pads. Sometimes in the wet your rotor or pads can get contaminated and that causes the brakes to squeal a bit. I would clean the rotor off with a disc cleaner and remove the brake pads (very easy to do) and give them a wipe.

I dunno what kind of disc brakes you have, but if they are hydraulic perhaps they are squooshy because they need a bleed.