You read the GPIO. If there is a high voltage (3V3) it will read 1, if there is a low voltage (ground, 0V) it will read 0.

Pin 26 is (Broadcom) GPIO 7.

Ensure that you do not feed a voltage greater then 3V3 into a Pi GPIO. You will likely damage the GPIO and/or the Pi itself.

There are several Python GPIO libraries.

See http://elinux.org/RPi_GPIO_Code_Samples#Python

from the docs

The code below should provide the functionality you are looking for.

import RPi.GPIO as GPIO

channel = 11
GPIO.setmode(GPIO.BCM)  
# Setup your channel
GPIO.setup(channel, GPIO.OUT)
GPIO.output(channel, GPIO.LOW)

# To test the value of a pin use the .input method
channel_is_on = GPIO.input(channel)  # Returns 0 if OFF or 1 if ON

if channel_is_on:
    # Do something here

According to https://www.kernel.org/doc/Documentation/gpio/sysfs.txt some of the GPIO can be configured as interrupt generating.

"value" ... reads as either 0 (low) or 1 (high). If the GPIO is configured as an output, this value may be written; any nonzero value is treated as high.

If the pin can be configured as interrupt-generating interrupt and if it has been configured to generate interrupts (see the description of "edge"), you can poll(2) on that file and poll(2) will return whenever the interrupt was triggered. If you use poll(2), set the events POLLPRI and POLLERR. If you use select(2), set the file descriptor in exceptfds. After poll(2) returns, either lseek(2) to the beginning of the sysfs file and read the new value or close the file and re-open it to read the value.

"edge" ... reads as either "none", "rising", "falling", or "both". Write these strings to select the signal edge(s) that will make poll(2) on the "value" file return.

This file exists only if the pin can be configured as an interrupt generating input pin.

Should be like this..

echo 17 > /sys/class/gpio/export
echo rising > /sys/class/gpio/gpio17/edge

Based on your comment, I recommend to check pigpio where a Java wrapper exists. The library​ is maintained by another user of this community. The wrapper is also well documented. I think with this you can develop bit more efficient.