Get yourself a hydrometer, it's a device that measures "specific gravity". You can get a cheap one for few bucks, but a good quality hydrometer shouldn't be more than $15 -- for your purposes I think you'd want one of high quality.
Ideally, the solution was well mixed (not stratified) when the batteries spilled, and you'll just need to match the specific gravity of whatever electrolyte is still in there. Get some battery acid with a specific gravity of 1.300, and mix it in a container with distilled or deionized or demineralized water until the specific gravity matches what's in the battery now. Specific gravity in all cells needs to be equal or as close as you can get.
If you can't get a good measurement, or aren't comfortable with the numbers you get, the battery manufacturer can probably tell you the recommended value for a fully charged cell.
Wet cell deepcycle motive batteries with solid lead positive plates are typically 1.267 to 1.284; if they have tubular positive plates, typical values are 1.300 to 1.310
Not sure where you'll find the acid -- I'd probably try an Interstate Battery store.
You probably know about handling that acid -- you can't be too careful -- it eats right through blue jeans (don't ask me how I know), and does it burn?