Tools Intro
Exploration sites cannot be found using the system scanner. You will need to probe down the sites using a variety of different probes. Ship, Skills and Rigs are also an immportant part of exploration.

Probe Types
Multispectral, Gravimetric, Magnetometric, RADAR and LADAR

The later 4 types each have different probes with different ranges and sensor strengths. These will be needed to hone in on a signal. These additional types are:

[Type] Quest Probe - 4 au range, 250 points primary sensor strength, 50 points other sensor strength, 2.088 au max scan deviation, 4000 sec flight time.
[Type] Pursuit Probe - 2 au range, 500 points primary sensor strength, 100 points other sensor strength, 6,250,000 km max scan deviation, 2000 sec flight time.
[Type] Comb Probe - 1 au range, 1000 points primary sensor strength, 200 points other sensor strength, 125,000 km max scan deviation, 1000 sec flight time.
[Type] Sift Probe - 0.5au range, 2000 points primary sensor strength, 400 points other sensor strength, 2,500km max scan deviation, 500 sec flight time.

Multispectral Frequency Probe - 999 au range, 1,000,000 points sensor strength (all types), 600 sec flight time

Ships
Covert Ops ships such as the Helios have a bonus that gives 10% reduction to duration/activation time of modules requiring Astrometrics per level, plus has -98% to -100% reduction in Cloaking Device CPI use per level - so if you are exploring in low sec space you can probe undetected.

Rigs
Gravity Capacitor Upgrade I - Reduces scan probe scan time by 10% ea rig.
Gravity Capacitor Upgrade II - Reduces scan probe scan time by 15% ea rig.

Skills
Signal Acquisition - 10% faster scanning with probes per level.
Astrometric Triangulation (Requires Astrometrics III) - 5% scan strength bonus per level of skill.
Astrometric Pinpointing (Requires Astrometrics IV) - 10% reduction in maximum scan deviation

Faster you can scan... faster you can see if there is something worth exploring and move on if there isn't.