If you do not have a tire pressure gauge, its going to be hard getting the exact PSI you need, however, you are not out of the fight yet.
If you have a little patience and good feel for your bike, you can probably get really close to the correct tire pressure.
Test by Feel – Although its not an exact science, you can actually judge your bike tire pressure by the way your bike feels. In fact, when I am out on the trail without a tire gauge, I often adjust my tire pressure based on the feel of the bike. If your bike feels sluggish and you keep banging the rim, you definitely need to add air. On the flip side, if you are bike feels to bouncy or jittery and you lose traction, you will need to let some air out of those tires.
Thumb Check – An age-old trick handed down from generation to generation is the ole “Thumb Check”. This involves filling the bike tire with air and then pinching the tire just above the rim to feel how tight it is. This is not too reliable and usually follows up a visually sag check, however, its been around this long for a reason.
Puddle Trick – Experimental* – Although I have not personally tried this, some riders have recommended the “Puddle Test”. This involves riding through a puddle to see how high the water comes up on the tire walls. If it is too high, it means your tires do not have enough air in them. I am a little suspicious on the physics of this one, so let me know if you have any luck.
Regardless of how creative you are, it can be a pain checking your tire pressure without a gauge.
To save you the trouble of having a gauge and a pump, or not having one at all, you can actually get a bike tire gauge and pump combo.
I recommend the Bell Airstrike 850, not only is this a low profile pump, it also fits Schrader and Presta valve tires.