Welcome to Ribnet!
I've always used a press of the thumb. There are others here would disagree, but if you do go pressure guage, pressure values should be in the book of words...... Which someone on here is bound to have a copy of if you don't.
Just remember if you pump it up ashore, the cold water will deflate it slightly when it hits the water. Likewise if you pump it hard on the water then leave it on a hot sandy beach for a couple of hours........