I think Geghis has the best ad hoc method. I recall that long wet grass can have a dramatic effect on takeoff run required, an increase of up to 30% may occur. (CAA Safety Regulation Group Accident Survey study 1991)
If you can't walk the length of the strip without squelching, or if water starts to seep up beside your foot when standing still it's definitely best to leave the aircraft parked.
If you really have to make the attempt (in a light aircraft, I assume), try taxying the full length before takeoff. If you don't bog the wheels then you have to make the difficult decision whether to go or not. If you bog it, the decision's already made!
It's a cliche but "takeoff's are optional, landings are mandatory."