For RNAV approaches there are a few approach minimums possible (applicable for WAAS systems only). These minimums will be in the minimums section towards the lower part of the approach chart. LPV, or Localizer Performance with Vertical Guidance, has the lowest minimums which are normally around 200 to 250 ft. The next higher minimum is LNAV/VNAV which is Lateral Navigation and Vertical Navigation. Then there is the non-precision LNAV minimum which is just Lateral Navigation and has a Minimum Descent Altitude (MDA) with no vertical guidance. However, Garmin WAAS systems will annunciate a LNAV+V for most LNAV approaches.
So what is LNAV+V and how do you know if an approach will have it? Garmin defines LNAV+V as Lateral Navigation with Advisory Vertical Guidance. This is an LNAV approach with an advisory vertical guidance, which is usually in the 3 degree range and is provided to assist the pilot in maintaining a constant vertical glidepath. Because it is advisory in nature and not an approach minimum the pilot is responsible for maintaining approach step down altitudes and obstacle clearance. Even considering this, it is a very handy feature that makes even a novice IFR pilot look like a pro.
And how can we tell if an LNAV approach will have the +V? First of all, if an approach has the LPV or LNAV/VNAV minimum the system will prefer these types as they will get you lower on the approach so LNAV+V will not be available. For straight LNAV approaches you can tell whether you will receive the +V when you load or activate the approach. In the GNS 430W/530W series units the LNAV+V will be in the procedures pages at the bottom when loading or activating the approach. In the GTN 750/650 series units the label of +V will not appear until the approach is loaded into the flight plan and then it will show in the title of the approach. G1000 systems will also display the +V in the title of the approach on the flight plan.
I hope this information has helped answer your questions on the LNAV+V and provided some guidance on how to find out in advance if the approach will have the +V available.