Yes, there is. It is conceptually very simple and there may even be an easy path to being able to do it conveniently. What you want to determine is the "titratable acidity". This is done by measuring out a known amount of the mash and then incrementally adding base of known strength to it until a specified pH is reached. The challenges are:
1) measuring out the proper amount of mash,
2) obtaining standard strength base
3) measuring out the base in increments accurately
4) determining when the target pH is reached.
Obviously, a balance is the best way to handle 1) but if all you are interested in is relative strengths and you intend to determine how much to add to a main mash based on volume a volume measurement might do. You'd mix up the mash thoroughly and draw off, say, 50 mL into a small beaker.
WRT 2) - vintners like to measure the titratable acidity of their musts and so kits are sold for measuring it. These contain sodium hydroxide solutions of known strength. The only problem I see with this is that once the stuff is exposed to the air it starts taking up CO2 which reduces its alkalinity so that over time its strength declines.
For 3) - the wine maker's kit must contain some means of measurement perhaps as simple as an eyedropper with the measured acidity expressed in terms of the number of drops it takes to reach the end point pH. The calibration may be something like "as citric acid per drop" but again, if you are only interested in relative values that may be sufficient.
WRT 4) - A pH meter is the obvious solution but again the kits must contain something that allows the end point to be detected. I don't remember what this is but I can't fathom it being an indicator dye as I would think the color of the must would mask the color of the dye and I can't imaging that it would be a test strip as they are so inaccurate.
At this point my suggestion would be to go to your LHBS (assume they also serve vintners) and ask if they carry acidity kits.