Well, I don't know how deep your pockets are, but probably the best tool is a network analyzer. You can sweep the antenna/feedline and get all the vital stats like return loss, SWR, impedance, etc. You can make the cool SWR graphs that you see in all the antenna manufacturers' catalogs, too.
Expect to pay $500ish for a basic used unit that will do HF/VHF frequencies. After that, the sky is the limit. You can get a nice used one for about $2k probably. HP and Anritsu are the biggest manufactures, but there are others like Wiltron who are good as well.
Of course, you can get by with other tools. The MFJ jobs are OK if you don't need real accurate readings. Another handy tool (that you could build yourself) is the dip meter. It will let you find the resonant frequencies of L-C circuits. RF millivolt meters, vector voltmeters, signal generators, etc can all be helpful at some point or another as well. Of course by the time you get all those you might as well have gotten the network analyzer.
I would start with a good multimeter, and a power meter that will measure reflected power or give you a direct SWR reading. This will allow you read return loss and see if your design is basically working.
Antenna design can be a lot of fun, and I wish you luck on your journey. Let me know if you have any questions.
Jeff