How do I check if values in a dictionary all have the same value X?

I will assume you meant the same value:

d = {'a':1, 'b':1, 'c':1}
len(set(d.values()))==1    # -> True

If you want to check for a specific value, how about

testval = 1
all(val==testval for val in d.values())   # -> True

this code will most often fail early (quickly)