Here is a list of resources I use to determine weather potential:
NWS - US National Weather Services
I use it to see what kind of weather expected in 3-5 days in general. Is there are any rain or storm expected, temperature trend. There is also Radar and Satellite Images available, which tells you how storms moving, where and what kind of clouds in the area.
RASP San Diego
RASP Santa Barbara
RASP Sierra Nevada
RASP - Regional Atmospheric Soaring Prediction. This is main XC planning tool, which is available online and shows you distribution of cloud base, wind speed and direction, convergence lines, expected cloud development and overdevelopment potential across region. The most important parameters are listed below:
Hcrit - is basically cloud base, MSL. For good flight you need at least 3000 ft. over terrain. Of course, you can fly with lower cloud base, but I don't count on a good result in this case and don't take a vacation day in the middle of the week.
BL Avg. Wind - if cloud base looks good next thing to check is a wind direction and strength. For PG wind strength in forecast should not exceed 10 knots, because in reality you can expect double of predicted wind speed due to addition of thermals, valley wind, compression and other factors. Depending on wind direction you need to determine launch point and anticipated route. After this step you need to come back to previous parameter and make sure that Hcrit is good along entire route. Keep in mind timing factor, condition changes along the day, so you need to know when and how wind direction will change. Of course, it's very approximate in forecast, but you must know at least general trend.
BL Max. Up/Down (Convergence) - If you anticipated route doesn't match to convergence lines then you need to reconsider your route one more time.
CU Cloudbase where CuPotential>0 - Useful to know where clouds will be forming and how high they're expected.
CU Cloudbase where ODpotential>0 - Overdevelopment is common on a good days, so be aware.
This is good tool to visualize GFS and NAM forecasts. You need to register (free) to create your own custom spots and forecast set. I use it to see wind speed, direction, precipitation and cloud cover (high, mid, low) on launch spots. It also calculates correct temperature on launch, taking into account it altitude.
NOAA Ready - National Oceanic and Atmospheric Administration
I use it to see sounding and stability, based on NAM 12 km, NAM 40 km and GFS forecasts. Stability is similar to combination of RASP Thermal Updraft Velocity and Hcrit, but calculated for one point and shown as graph in time. Sounding helps to determine thermal and wind strength, wind direction, depending on altitude. Even points of interest are similar to RASP, sounding provides raw data for all important parameters, when RASP shows you only average values. NOAA Ready is very useful for verification of flight concept made based on RASP.
NWS weather history
Before going to fly it's useful to check current wind and past day weather history. In the link above you can change KBUR code to other airport for example, KSBA - Santa Barbara, KVNY - Van Nuys, KSBD - San Bernardino.