-Your feet keep slipping. This is gravity's way of telling you your butt is too high and you need to bring it down.
-You don't feel anything in your abs. You should engage your core during your plank. It's hard to have poor form while doing so, but if you don't feel anything in your abs, drop your butt down until you feel it hit your abs.
-Your lower back hurts. Most likely, your back and butt are sagging. In this case, pick your butt and hips up until it leaves the back and hits the core.
-You can see your feet. You should have a neutral head/neck. This means your spine should be straight from your butt all the way up to your head. People have a tendency to tuck their chin to their chest and look underneath them towards their feet. This is no good for the neck. Your eyes should form a line straight down to the ground, directly between your wrists.
-You stress your shoulders. Your shoulders do play a role in supporting your body, but be careful not to over stress them. Take a look at where your elbows are. You elbows should be directly below your shoulders. If they're in front of you, you need to shift your weight forward. The same goes for the pushup position. If your hands are not directly below your shoulders and are in front of you, you need to shift your weight forward until they are in the correct position.
In addition to these signs to pay attention to, always focus on flexing your abs and your glutes (butt). It's extremely hard to sag your back or stick your butt way up in the air when these muscles are engaged and will help minimize the risk of future back pain.