How Can I figure out my Effective Tone Arm Mass??

I was wondering if there is a way for me to be able to determine this as I do not have any info on my tone arm. Thanks.
Mount a cartridge and play the resonance test tracks on the HFN test record. This will give you the horizontal and vertical resonance frequencies of that particular cartridge/arm combination. If you know the mass and compliance of the cartridge, you can calculate the effective horizontal and vertical mass of the arm. (They're often different.)

I'm too rusty on the math to remember the formula, but someone here or on VA will certainly know it.
Effective mass calculations are not only considered in the initial tonearm design criteria, but also in the weight of the raw tonearm components. Also, effective mass is established in the static and axial weighting of all components comprising the tonearm assembly. Tonearm mass and cartridge compliance go hand in hand.

What tonearm are we discussing?
A Systemdek Profile tonearm on a Systemdek llX TT. It will go eventually but right now I want to get the TT going as best I can. I have 2 cartridges; A Grado Sonata and a Denon DL103. The first is light and the Denon is heavier. So I am trying to figure out which one would seem to be a better match.
You can use the method Dougdeacon described and look up the chart at the bottom of the FAQ posted on the VDH web site. This FAQ also contains over 200 Q&A covering almost everything related to vinyl playback, some of them are quite insightful.
Great info, thanks!
The Profile arm is an OEM model made by Sumiko in the 1980s, for the lower cost audiophile market. It is a medium mass tonearm, and the bearings are designed for mid-compliance type cartridges. Therefore it would be better matched to the Grado, because the DL103 will chew the bearings up in that arm. You have to have top grade bearings with close tolerance adjustment for a 103. However, it may be not be the greatest match with the Grado either. It could hum because the Systemdek has the motor under the platter, which is known to induce hum in Grado cartridges. I'd suggest a Goldring 1042 or a Dynavector 10x4 or 10x5 for that tonearm.
Thank Twl! Yes, there was some hum with the Grado, but I resolved it thanks to everyones help with my earlier question on that. When the time comes do you think I should get a new arm or arm and table? I guess I know the answer to that though :-(
The Systemdek was a pretty good table for the money, but they have been out of business for a long time. The newer Audio Note turntables seem to be a clone of the old Systemdek, and I suppose it is possible that even some parts may be interchangeable, but I'm not sure about that.

When it is time for you to upgrade, then I recommend doing it in the proper order of importance: table, arm, cartridge.

If you'd like, you can contact me when the time comes, and I'll make some suggestions.