If you want to test them before hooking up drivers and to an amplifier to ensure you have wired them correctly (so you don't short and blow your amplifier), then connect your drivers to your crossover and use a multimeter to measure the DC resistance. If > 3 Ohms you'll be ok.
You can't really measure crossover performance without the drivers intended attached. A crude way would be to put a resistor in place of each driver rated at the drivers nominal impedance. ie. if you have an 8 ohm nominal woofer and 8 ohm nominal tweeter, put an 8 ohm resistor in place of the woofer and another in place of the tweeter.
If you are measuring the transfer function, it depends on the software you are using.
Thank you, that answered my question. I do have access to a scope. Listening of course is a fair test but since these were built from plans unless I perform a more objective test i wont know if I built them correctly. Incidentally, can cross overs produce much heat?
Yes crossovers can produce a lot of heat. I've melted inductors before so be carefull. See if you can trace where the signal goes and this is the path of most heat. In my case it the the leg that shorted out the high frequencies in the low pass woofer filter.
To your original question, the real speaker load has a major effect on the crossovers response. you also have to be careful about grounds as most test equipment is grounded on one terminal and not all amplifiers allow for one of there outputs to be grounded. You can blow up a lot of amps this way. They can oscillate at frequencies that you can't hear and just over heat without your even realizing it.
i wasn't sure as to what type of test you were looking for if you are looking to test to see if you built them correctly do the test that dave stated the oscope and audio gen. will give a rough idea if the cut off freq. after that they will still need to be hooked up to get them just where you would like them to be . also gedlee is correct .just remember the test i stated should be done after you know they are electrically correct. i use it for initial set up in car audio . well good luck and have fun with your project.
These days of cheap sound cards and virtually free software there is no reason not to be using the more sophisticated measurements like transfer functions. A little practice and very complex electrical measurements become easy. Just lookup "software spectrum analyzers" on the web. In fact Sigview looks so good I may buy it!
Measuring complex impedances, like speaker loads, is trivial with a two channel analyzer and a small (I use .1 ohm) sense resistor (to measure current). Take the amp output as channel 1, the voltage across the load resistor (the current) as channel 2 and the impedance is then the tranfer function of these two signals channel 1 divided by channel 2. Scale the measurement by using a know resistor value. Just be careful of ground points. The sense resistor has to go on the ground side of the circuit.