Prepare 1 liter of 28% NH3(aq) solution from 1 gal of %10 NH3(aq)
Disclaimer: ammonia is extremely hazardous including flammable in some vapor concentrations.


  How do I know when I have reach %28 NH4OH concentration?


You can measure the specific gravity and compare to the table on page 2 of 
this document:

Next, it is important to understand that if you heat/boil/distill the 10% solution the vapor coming off is not 100% ammonia, it is a mixture of ammonia and water, but with increased ammonia concentration. 

To predict the percent ammonia in the vapor phase, you need to look at the dew point line in a table like Fig. 1.2 here, but that one is for 16 bars or Fig. 2 of Ganesh et al. Journal of Mechanical Engineering Research Vol. 3. (1), pp. 25-39 that shows many pressures including 1 bar.

As the original solution is heated, the system progresses toward the upper left in Fig. 1.2/Fig.2, the liquid phase along the bubble point line and the vapor phase along the dew point line.  The concentration of ammonia in both phases progressively decreases.  Eventually, the concentration of ammonia in the vapor phase drops below the concentration of ammonia in the original solution.  

From Fig. 2 of Ganesh et al. it can be seen that at 1 bar starting from 10 weight% ammonia liquid phase, the initial vapor phase concentration would be close to 28% ammonia.   

This shows that even if you added no water to the condensate, there is no way you can get 1 liter of 28% ammonia from 1 gallon of 10% ammonia in one simple distillation step.  

Only through fractional distillation could you get that much 28% ammonia from that little 10% ammonia.  

Also, practically speaking, it will be difficult to avoid lossing ammonia vapors to the environment and safety needs to be considered.