Like Edward above, everytime I travel I weight check on the first dive - even though the items that will be different from my normal weekly diving are cylinder, and perhaps exposure suit. Like Edward my weightign decisions are instinctive based on experience, to teh following is used for conformation
My rule of thumb, is with a full tank, normal breath etc, having deflated my BCD, to "float" with the top of my head about a hands width under the surface. That gets me into the ballpark and I'll be beside the boat if I need to add or ditch
During the dive and especially near the end I'll mentally assess my buoyancy (often feeling the wing to see how much air its got (not really possible in a jacket) and then make any changes as required
Always remember that on the first dive the wetsuit will be dry thus slightly more buoyant, and newer/infrequent divers will need a few diver to relax and thus will be slightly more buoyant. So during a multi day trip you could expect to reduce yoru starting weight a little further.