A pull up training pant will likely leak before it gets wet enough to be squishy so the best way you can check to see if a Pull Up is wet is just but removing your child's pants and taking a look inside the Pull Up training pant to see if they've wet the Pull UP.
If they wet the Pull Up even just a little bit then you need to change the Pull Up so they won't get used to being wet and potty train sooner.
If you leave the child in even a slightly wet Pull Up then they might get used to being wet and take longer to potty train.