Objective ones. Like the actual sources... and myths... and looking for the comparisons myself.
There is a christian apologetics site that makes a list of all the comparisons and then also lists the sources used - I pay attention to only the non-biased ones. Plus a little common sense...if someone (like Zeitgeist, or the pagan christ authors, or any such thing) has to pad their case... they either do not have a case, or they are simply very shoddy and biased researchers. So why believe anything they have to say? When someone lists a big list of all sorts of 'god-men' stating that one of the main similarities is a dec.25th b-day, that should be a red flag as to the authenticity of their research right off the bat.