Unfortunately that information is not always provided by manufacturers, and when provided it's most of the times very incomplete. However you can start with some of the multisync presets, like the WG D9800, and tweak it if necessary.
BTW which brand/model is it?
These are the lines required for WG D9800, you just substitute the ones in mame.ini with these ones:
monitor_specs0 15250-18000, 40-80, 2.187, 4.688, 6.719, 0.190, 0.191, 1.018, 0, 0, 288, 448
monitor_specs1 18001-19000, 40-80, 2.187, 4.688, 6.719, 0.140, 0.191, 0.950, 0, 0, 320, 448
monitor_specs2 20001-29000, 40-80, 2.910, 3.000, 4.440, 0.451, 0.164, 1.048, 0, 0, 384, 576
monitor_specs3 29001-32000, 40-80, 0.636, 3.813, 1.906, 0.318, 0.064, 1.048, 0, 0, 576, 768
monitor_specs4 32001-34000, 40-80, 0.636, 3.813, 1.906, 0.020, 0.106, 0.607, 0, 0, 576, 768
monitor_specs5 34001-38000, 40-80, 1.000, 3.200, 2.200, 0.020, 0.106, 0.607, 0, 0, 600, 768