Assuming that this is not your first adventure into wine making (Based on number of previous posts) Hopefully you have some of the basic tools. Unfortunately the folks who sell starter kits to consumers leave out any pH testing tools.
Even a simple package of pH test strips would work for white or light colored wines. So you have at least 3 ways to test the acidity of your wine in the making.
1) pH test strips - you need some made for wine that will measure between 3.0 and 3.8 That would let you know if your wine is within the correct range. The problem is that test strips require you to interpret the color results and that ranges from slightly difficult (white wines) to impossible (Red wines or for the color blind). (Cost -$3.00-$10.00)
2) A TA test kit. They work great but remember that the sample, though small, becomes toxic after the test and has to be tossed out. They are very accurate and relatively easy to use if you follow the included directions. (Cost - $8.00-$15.00)
3) A pH meter. Easiest to use and you can test as often as you like without losing the test sample or running out of material (Test strips or Chemicals) like 1 & 2. You do have to check calibration on them peridically as they can drift off. They require basic care and recalibration solutions to re-calibrate but other than that they are probably the easiest test tool to use. (Cost -$20.00 - $ Hundreds (for the high-class an swell varities)) With a pH meter you can also do a TA test with the most accurate results but again that would require an additional chemical and render the sample toxic just as in thee standard TA test process.
Don't forget that pH and TA test acidity slightly differently but the end results you need are - Is my wine acidic enough to keep or is it over the top acidic and need to be toned down. (If you need help understanding the difference between TA and pH look through the threads on this forum - there are some excellent posts to help you underdstand.
One last point - some folks test both, some only do pH, some only do TA, and some don't test at all. The choice is yours and there is no wrong answer as long as you know the possible end results of your choice.
) as a lot of us do instead of introducing different flavors, your alcohol level will drop a little but as you said by shooting a little higher that final ABV will remain high enough to help prevent spoilage. At least that's the way I run my batches. I like to keep my wines 100% of one variety of fruit whenever possible unless I plan a mix or additive flavor before starting. Most times I'm adding no more than 8-12 ozs throughout the process so dilution is not a serious issue.
