Famous non-profits like the Salvation army are obviously reliable and the money and resources provided by them come from trusted donors.
But thoroughly go through the non-profit institute’s website and the documents (especially if you have applied for a loan, carefully check the particulars like the rate of interest, time period of repayment and the other terms and conditions for the same) before signing any papers.
Take second opinions or try to get in touch with someone who has already availed of the particular window replacement grant program’s services and find out their experience with the particular institute that you are looking into.