Verifying line of sight starts with a trip to your rooftop. Things can look surprisingly different from that vantage. If your distance is under five miles, then a good microwave path would be at least fifteen feet above the (mature) tree line and would see at least an eight to ten foot radius of the remote site. Best targets are elevator penthouses, which make ideal locations for wall mounts.
If you don't have line of sight, but you think it's close (within 10-15'), then a commonly effective method is fill a six to ten foot weather balloon (find it on-line) with helium and float it above your rooftop. Measure the string at the point where someone at the remote site confirms that they can see the balloon. This is a less precise method when drags the balloon sideways and so reasonable allowances must be made.
Keep in mind that there are practical limits as to how high you might place an antenna on your roof and generally that's about fifteen feet. Higher than that and the antenna mount goes from a simple pipe or tripod to a more substantial "stub" tower. At that point, you'll probably need a windload analysis to determine that the structure can support the tower. Another concern may be aesthetics. If a rooftop tower is out of the question from that standpoint - and likewise, a ground based tower or monopole, then it becomes necessary to find a repeater site.
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