Just that . I am not too much concerned about the accuracy of measurements .
It depends what you mean by "without an instrument". You need something to measure pressure with - your own senses won't be able to measure everyday pressure changes (though they could measure the change if for example you were thrown out of an airlock into space without a space suit).
You can easily build a diy barometer. Just take a glass bottle turn it uside down and put the neck into a bowl of motor oil.
When the atmospheric pressure increases, the oil level in the bottle will rise, and when the pressure decreases, the oil level in the bottle will fall. This happens because the mass of air in the bottle is fixed, so an increase in atmospheric pressure reduces its volume and vice versa.
You can use any fluid, but water will evaporate into the bottle and air in the bottle will dissolve in the water. I suggested motor oil because it's vapour pressure is low and air isn't very soluble in it.