We'll look at several spectra to illustrate different strategies for interpretation.
Example 1. HRMS.
Several different molecular formulae have a "nomimal mass" of 98. When measured to 6 significant figures, they reveal slight differences based on the exact masses of different atoms:
			  C7H14:  98.1096

			  C6H10O: 98.0732

			  C5H6O2: 98.0368

			  C5H10N2: 98.0845
The Journal of Organic Chemistry accepts observation of a HRMS signal within 0.003 mass units as evidence of composition of a new compound, though elemental analysis is still preferred to establish both molecular formula and purity.
Example 2. 1, 2, and 3-carbon hydrocarbons.
Methane
CH4Mass.jdx 		Methane has the following signals and relative intensities: 
Mass/Charge   Intensity
12            3.80
13           10.69 
14           20.42 
15           88.79
16          100 
17            1.64 (theoretically:  should be 1.1)
Acetylene
C2H2Mass.jdx 		Acetylene has the following signals and relative intensities: 
Mass/Charge    Intensity
12              0.70 
13              3.20 
14              0.10 
24              5.00
25             19.12 
26            100 
27              2.20 (theoretically: should be 2.2)
28              0.10
Propene
PropeneMass.jdx 		Propene has the following signals and relative intensities: 
Mass/Charge    Intensity
12                0.90 
13                1.40
14                3.40 
15                5.50 
16                0.20 
19                2.20
20                1.80 
21                0.10 
24                0.30 
25                1.80
26               10.51 
27               38.73 
28                1.40 
36                1.80
37               12.71 
38               19.42 
39               72.57 
40               29.13
41              100 
42               70.36 
43                2.30 (=3.27% of m/z=42; theoretically 3.3%)
44                0.10
Benzene
BenzeneMass.jdx 		Benzene has the following signals and relative intensities: 
Mass/Charge    Intensity
49              3.92
50             20.82 
51             22.11 
52             18.84 
53              0.77
61              1.02 
62              1.10 
63              4.97 
73              2.46
74              6.22 
75              2.25 
76              5.89 
77             28.37
78            100 
79              6.53 (theoretical:  6.6)
Example 3. Halogens Cl and Br are evident from intense (M+2)+ peaks.
Chlorobenzene
PhClMass.jdx 		Chlorobenzene has the following signals and relative intensities: 
Mass/Charge    Intensity
112             100
113               6.91 
114              32.93 (theoretically 32.5) 
115               2.10
116               0.10
Dibromobenzene
PhBr2Mass.jdx 		For a predictive algorithm, for to the this site to calculate the isotopic distribution for a given molecular formula.