Buy yourself a bicycle floor pump with a gauge on it. Doesn't need to be a expensive one. I've used a pump I bought at Walmart for $10 USD back prior to 2010.

Also if you search for bicycle tire pressure gauge, you might find some that go to higher pressures. I guess you are using Schrader valves since you say you've filled at gas stations. Consider getting a gauge that does both presta and Schrader.

The tire isn't going to suddenly explode if you go over the max tire pressure. Just don't ride it over the max pressure. You should be riding it at your particular ideal pressure which is unlikely to be max pressure. I've inflated tires to 150% of their max pressure when trying to seat a tire that doesn't want to seat. I've only had issues with the tire blowing off the rim when I go over 150% of the max.

Keep your hands clear though. It hurts like heck when they blow off and that tube slaps your hand.