how do you find the frequency that a transmitter and receiver are using?

I know the transmitter I am using is sending out out on 433mhz but the receiver is not responding. What tool do you use to test frequencies to make sure I have them both set to the right frequency?

What sort of Transmitter and Receiver are you using. When you say the receiver is not responding, what do you mean. How are you looking at the receivers output. Actually measuring the Transmitters frequency needs either a frequency counter or spectrum analysis tool which most people dont have .

Got a multiband/shortwave kind of radio? Tune to 433 Mhz, code up something to put out bursts of tone, see if you can tune it in. Those inexpensive Tx/Rx modules are pretty robust. You can set one of each up on two Arduinos and use Virtualwire library to send data across. Use 17cm antenna on each, just a simple piece of wire.

Got a scanner. Program the scanner for your freq and see if you hear the carrier. Those eBay 433 MHz transmitter and receivers work pretty well. You should easily be able to hear the carrier with a scanner.

A frequency counter is another way to tell what frequency a transmitter is operating on. A RF signal generator is the only instrument I can think of that one can find out what frequency a fixed frequency receiver is tuned to, by generating and tuning across the frequency spectrum and see which frequency the receiver responds to.