[horseb0x]

The obvious way would be to look for the parent peak (the peak caused by the unfragmented molecular ion) at the end of the spectrum but how can you tell the parent peak from one caused by heavier isotopes? Also how do you know if theres a parent peak at all and that all the peaks aren't fragments? Are there any tricks I should know about for finding the molecular weight from the mass spectrum?

[Borek]

how can you tell the parent peak from one caused by heavier isotopes?

Intensity.

[dennyman100]

The most intense peak is not always the parent peak, it is just the most stable radical formed when the sample is fragmented.  If the sample contains a Cl ion then there will be a M+2 peak with about 1/3 the intensity of the M peak.  If the sample contains a Br atom then the M and M+2 peak will be about the same intensity.  Also different isotopes come into affect for the relative intensities.