Using if statement to find where two variable columns are same and storing index

I'm trying to store the indices where two DataFrames have the same values. For example...

current_gages = 95x4 DataFrame with the following headers: site_no, sitename, Lat, Lon

ucol_gages = 253 x 2 Dataframe with the following headers: site_no, area

I want to find where the indices for these two dataframes have the same site number (site_no).

I am working with the following:

count1=0
gages_idx=[]
for j in range(len(current_gages)):
    if ucol_gages['site_no'][count1] == current_gages['site_no'][j]:
        gages_idx.append([count1, j])
        count1+=1
    elif ucol_gages['site_no'][count1] != current_gages['site_no'][j]:
        count1+=0

The loop stops at [4,4] with the first if statement and then to make sure there aren't any other rows in current gages, I added the elif statement to run through the remainder of the rows.

I know the next indices where ucol_gages and current_gages match is: ucol_gages['site_no'][5] and current_gages['site_no'][4].

Is there an easier way to do this? I need to make sure I run through all variations of the row indices within the DFs to determine any possible matches.